Run every lint in a container via Dockerfile.lint (closes #40)

script/lint runs the linter directly when it is already inside a
container and otherwise builds Dockerfile.lint, so the linter never runs
on a developer host. That closes three host-only mechanisms: the result
cache golangci-lint keys on file content rather than location, which
produced a confirmed false green and findings reported against other
checkouts; the host-global $TMPDIR/golangci-lint.lock, which fails a run
in a way no caller can distinguish from findings; and host/container
version skew, which hid thirteen findings on one repo.

Detection is on LINT_IN_CONTAINER=1, set by every Dockerfile, and on
nothing else. The two directions are not symmetric: a false negative
inside a container attempts a nested docker build, finds no daemon and
fails loudly, while a false positive on a host silently lints there,
which is the defect this issue exists to kill. /.dockerenv is therefore
rejected even as a fallback -- measured absent inside BuildKit RUN steps
and present on any host that is itself a container, so it fails in both
directions and one of them is the dangerous one.

Nothing else changes shape. The Dockerfile still runs make check,
script/check still runs test, lint and fmt-check, script/cibuild is
still a single docker build with CHECK_EPOCH and VERSION, and the Go
multistage lint stage and its COPY --from=lint ordering dependency
survive with ENV LINT_IN_CONTAINER=1 added. Dockerfile.lint is the
standalone developer-host path and carries the same CHECK_EPOCH guard,
with the ARG below the dependency layer so only the lint re-runs.

The script/bootstrap golangci-lint install and the per-checkout
GOLANGCI_LINT_CACHE/TMPDIR wrapper are deleted as superseded. Neither
has a caller left. A JS repo's yarn install stays: the rule is that no
lint verdict may come from a host invocation, not that no linter binary
may exist there, and in a repo whose formatter is its linter the
formatter necessarily runs on the host.

golangci-lint config verify is kept, on measurement. Under the pinned
v2.12.2 a bogus top-level key and a bogus key under linters.settings.lll
both pass `golangci-lint run` with exit 0 and `0 issues` while config
verify exits 3 and names them; an unknown linter name fails run and
passes config verify. It needs no network: every case reproduced
byte-identically under `docker run --network none`, in a container where
`getent hosts golangci-lint.run` exits 2.

Comment blocks were cut hard across every file this unit touches.
.dockerignore drops from 67 comment lines to 28, script/cibuild from 17
to 12, script/docker from 18 to 12, and prompts/REPO_POLICIES.md from
1182 lines to 907. What remains says why a line is load-bearing; the
discovery narratives are gone.

config verify lives in script/lint's native branch rather than in a
Dockerfile, so every path that lints inherits it: the lint stage of the
main image, which is what CI runs, as well as Dockerfile.lint. Putting
it in one Dockerfile is how the other path silently loses it.

.dockerignore

@@ -1,75 +1,37 @@
-# Docker matches this file with moby/patternmatcher: Go filepath.Match
+# .dockerignore does NOT use .gitignore semantics. Docker matches with
-# semantics plus a `**` extension, compiled to a regexp. Plain
+# moby/patternmatcher: filepath.Match plus `**`, so `*` does not cross
-# filepath.Match has no `**` at all. What follows from that: `*` does not
+# `/` and an unprefixed pattern is anchored at the context root. Every
-# cross `/`, and a pattern without a leading `**/` is anchored at the
+# depth-independent pattern therefore needs `**/`, or `config/.env` and
-# build-context root. Every depth-independent pattern therefore needs the
+# `certs/server.key` still ship while the file reads as solved. Only
-# `**/` prefix — without it `config/.env` and `certs/server.key` still
+# genuinely root-anchored entries go unprefixed. Never transplant these
-# ship while the file reads as solved.
+# into .gitignore, where `**/` is wrong.
 #
-# Root-anchored entries are for paths that occur exactly once, at the
+# Matching is case-sensitive, so secrets use character ranges rather
-# context root. A host-built binary is the usual case, and it must be
+# than an ALL-CAPS twin, which would still miss `Server.Key`.
-# written anchored: `/myapp`, never `**/myapp`. The prefixed form also
-# matches `cmd/myapp/`, which deletes the package directory from the
-# context. In-repo agent scratch is the other case, for the same reason
-# — with the caveat recorded at that entry: anchoring is exact only
-# where agents run at the repo root, and a repo where they do not must
-# add its own entries.
 #
-# Matching is case-sensitive, so `**/*.key` does not match
+# Extend with this repo's own host-built artifacts, written anchored:
-# `certs/SERVER.KEY`, which is reachable on the case-insensitive
+# `/myapp`, never `**/myapp`, which also matches `cmd/myapp/` and
-# filesystems most laptops use. Adding an ALL-CAPS twin per pattern is
+# deletes the package directory from the context.
-# not the fix: it still misses `Server.Key` while reading as though case
-# were handled. Character ranges cover every spelling in one line, so
-# every secret name below is written that way — including the
-# extensionless SSH keys and `.envrc`, because on those same
-# case-insensitive filesystems direnv reads `.ENVRC` and ssh reads
-# `ID_RSA`.
-#
-# `**/*.[eE][nN][vV]` also excludes a committed env template such as
-# `example.env`. If the build genuinely needs one, re-include it with a
-# negation after the pattern: `!docs/example.env`.
-#
-# Extend this file with the repo's own host-built artifacts (compiled
-# binaries, test binaries, coverage output); those are per-repo and
-# belong here because a host build otherwise drops them into the
-# context.
 
-# Repository metadata: exactly one, at the context root. Excluding it
+# Excluding .git means `git describe` cannot run in any build stage and
-# means `git describe` cannot run in any build stage, and it fails
+# fails quietly there; pass the version in with --build-arg VERSION.
-# quietly there rather than erroring, so a version embedded that way
-# comes out empty. Compute the version on the host and pass it in with
-# `--build-arg VERSION=...`; see the version rule in REPO_POLICIES.md.
 .git
 
-# In-repo agent scratch: a directory holding a full additional checkout
+# Agent scratch: one full checkout of the repo per in-flight agent.
-# of the repo for each in-flight agent. Anchored because it occurs
+# Anchored because it occurs once where agents run at the repo root.
-# exactly once *where agents run at the repo root*, which is the
+# KNOWN GAP: a repo running agents in subdirectories still ships
-# convention this file assumes; the `**/` form would also match any
+# `services/api/.claude/` and must add its own anchored entry.
-# nested directory of that name and delete it from the build.
-#
-# KNOWN GAP, and it is not hypothetical: the directory is created in the
-# agent's working directory. If agents in this repo run in
-# subdirectories — a monorepo with a per-service agent, say — then
-# `services/api/.claude/` is NOT excluded by the line below and still
-# reaches the build context and the image, which is the exposure this
-# entry exists to close. A repo in that shape adds its own anchored
-# entries (`/services/api/.claude`), or `**/.claude` after confirming no
-# legitimately named nested directory would be caught.
-#
-# Not case-folded, unlike the secret patterns below: tooling creates
-# this directory in exactly one spelling, so a folded pattern would add
-# no coverage.
 .claude
 
-# Environment files. `*.env` covers both the bare `.env` name (`*` matches
+# Environment files. `*.env` covers bare `.env` and the `prod.env`
-# the empty string) and the `prod.env` convention.
+# convention. Re-include a committed template with a negation if the
+# build needs one: `!docs/example.env`.
 **/*.[eE][nN][vV]
 **/.[eE][nN][vV].*
 **/.[eE][nN][vV][rR][cC]
 
-# Private keys and the bundles that carry them. Public certificates
+# Private keys and the bundles carrying them. Public certificates
-# (*.crt, *.cer) are deliberately absent: they are not secrets and are
+# (*.crt, *.cer) are deliberately absent: they are legitimate inputs.
-# sometimes a legitimate build input.
 **/*.[pP][eE][mM]
 **/*.[kK][eE][yY]
 **/*.[pP]12
@@ -86,8 +48,7 @@
 **/.DS_Store
 **/Thumbs.db
 
-# Editor state. Never a build input, and it churns under a developer's
+# Editor state: never a build input, and it churns COPY.
-# hands, so it invalidates COPY for reasons unrelated to the source.
 **/*.swp
 **/*.swo
 **/*~

Dockerfile

@@ -3,26 +3,26 @@ FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e3
 
 WORKDIR /app
 
-# script/bootstrap installs all prerequisites (make via apk here; node
+# Makes script/lint run the linter directly rather than building
-# and yarn are already in the base image, so those steps are skipped).
+# Dockerfile.lint, which would need a docker daemon here.
-# Dependency manifests are copied first so the bootstrap layer is
+ENV LINT_IN_CONTAINER=1
-# cached until they change.
+
+# script/bootstrap installs all prerequisites. Manifests are copied
+# first so that layer stays cached until dependencies change.
 COPY script/ script/
 COPY package.json yarn.lock ./
 RUN script/bootstrap
 
 COPY . .
 
-# CHECK_EPOCH is a per-invocation nonce supplied by script/cibuild and
+# CHECK_EPOCH is a per-invocation nonce from script/cibuild and
-# script/docker. Without it an unchanged tree serves this layer from
+# script/docker; without it an unchanged tree serves this layer from
 # cache and the build reports a green it never ran. ARG is stage-scoped,
-# so it must be redeclared in every stage that runs checks. The guard
+# so declare it in every stage that runs checks. The guard fails a bare
-# makes a bare `docker build .` fail loudly instead of silently reusing
+# `docker build .`, which would otherwise reuse the empty (and therefore
-# the empty (and therefore stable) cache key. Expand the value into the
+# stable) cache key. The value is also expanded into the check command,
-# command so the cache miss does not depend on BuildKit's handling of an
+# so the cache miss does not depend on BuildKit's handling of an
-# unreferenced ARG. Both the guard and the check RUN reference the value,
+# unreferenced ARG; keep both references.
-# so both are value-keyed: there are two independent invalidation points
-# here, not one. Keep both.
 ARG CHECK_EPOCH
 RUN [ -n "$CHECK_EPOCH" ] || exit 1
 RUN echo "check epoch: ${CHECK_EPOCH}" && make check

Dockerfile.lint

@@ -0,0 +1,27 @@
+# Lint-only image, built by script/lint when it is not already inside a
+# container. Linting is a build step, so a successful build is a clean
+# lint, and nothing is bind-mounted, which matters when the daemon is
+# remote.
+#
+# node 22-alpine, 2026-02-22
+FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34
+
+WORKDIR /app
+
+# Makes script/lint run the linter directly instead of recursing into
+# another docker build, which has no daemon here.
+ENV LINT_IN_CONTAINER=1
+
+COPY script/ script/
+COPY package.json yarn.lock ./
+RUN script/bootstrap
+
+COPY . .
+
+# ARG sits after the dependency layer so that layer stays cached and
+# only the lint re-runs. The guard fails a bare `docker build
+# -f Dockerfile.lint .`, which would otherwise reuse the empty (stable)
+# cache key and report a lint it never ran.
+ARG CHECK_EPOCH
+RUN [ -n "$CHECK_EPOCH" ] || exit 1
+RUN echo "lint epoch: ${CHECK_EPOCH}" && make lint

README.md

@@ -117,7 +117,9 @@ alpine. We provide:
 - `script/projectname` — output the project name (our own extension); used by
   `script/docker` for the image tag
 - `script/test` — run the test suite (no tests defined here)
-- `script/lint` — lint the markdown files with prettier
+- `script/lint` — lint the markdown files with prettier. Inside a container
+  (`LINT_IN_CONTAINER=1`, set by both Dockerfiles) it runs prettier directly; on
+  a host it builds `Dockerfile.lint` so the linter still runs in a container
 - `script/fmt` — format all markdown files with prettier (writes)
 - `script/fmt-check` — check formatting (read-only)
 - `script/check` — run all checks: `test`, `lint`, `fmt-check` (our own

TODO.md

@@ -21,6 +21,56 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-08-10: Moved every lint run into a container. `script/lint` now runs the
+  linter directly when `LINT_IN_CONTAINER=1` and otherwise builds
+  `Dockerfile.lint`, so the linter never runs on a developer host — closing the
+  content-keyed result cache that produced a confirmed false green, the
+  host-global `$TMPDIR/golangci-lint.lock`, and host/container version skew.
+  Detection is on that marker alone: a false negative inside a container fails
+  loudly on the missing daemon, while a false positive on a host would silently
+  restore host linting, so `/.dockerenv` is rejected outright — measured absent
+  inside BuildKit `RUN` steps and present on hosts that are themselves
+  containers. Everything else keeps its existing shape: `make check` still runs
+  in the image, `script/cibuild` is still one build, and the Go multistage lint
+  stage survives with `ENV LINT_IN_CONTAINER=1`. `Dockerfile.lint` carries the
+  same `CHECK_EPOCH` guard, with the `ARG` below the dependency layer so only
+  the lint re-runs. The `script/bootstrap` golangci-lint install and the
+  per-checkout cache/lock/`.lint-cache` wrapper are deleted as superseded; a JS
+  repo's `yarn install` stays, since the rule is about where a verdict comes
+  from, not about which binaries exist. `golangci-lint config verify` was kept
+  on measurement: a bogus config key passes `golangci-lint run` with `0 issues`
+  and fails `config verify`, and every case reproduced byte-identically under
+  `--network none`, so the schema is embedded and the line costs no network.
+  Comment blocks across the touched files were cut hard in the same pass.
+- 2026-08-09: Made a golangci-lint result belong to the tree that asked for it.
+  REPO_POLICIES.md now carries the canonical Go `script/lint`, which gives the
+  linter per-checkout `GOLANGCI_LINT_CACHE` and per-checkout `TMPDIR`. The two
+  are separate defects and the second is the one that gets dropped: the result
+  cache is keyed on file content rather than location, so checkouts holding
+  identical files serve each other's findings under the other's path, while the
+  concurrency lock is `$TMPDIR/golangci-lint.lock` — host-global, independent of
+  the cache, and unaffected by isolating it. Moving workers from worktrees to
+  their own clones does not help either half; it only removes the foreign-path
+  artefact that made the defect visible. The lock error is retried rather than
+  surfaced, because it is not a result: it exits non-zero exactly as findings
+  do, and reporting it as findings sends a correct branch back for rework.
+  Detection is on the stderr stream and never on exit status, so a finding
+  quoting the lock message in source cannot be retried away, and exhaustion
+  exits 75 with a VOID message rather than passing or failing quietly.
+  `--allow-serial-runners` (which keeps the guard and queues) covers the
+  same-checkout overlap that `TMPDIR` scoping cannot; `--allow-parallel-runners`
+  is rejected outright. The stdout and stderr capture files are per invocation
+  rather than per checkout, because serialising the linter does not serialise
+  the shell's redirections: two runs in one checkout — the overlap the flag
+  exists to support — would otherwise truncate and read each other's output,
+  which is the same defect one layer above where it was fixed. Both checklists
+  gained the corresponding items, since a half-fix that sets only the cache
+  reads as complete. `GOCACHE` was measured and does not need isolating.
+  Verified with the snippet extracted from the committed document and executed
+  as a consuming repo would adopt it, against paired controls: contamination
+  reproduced on the pre-fix form and absent on the adopted one, retry engaged,
+  exhaustion loud, a genuine finding still reported, and a held host lock
+  failing the pre-fix script while leaving the adopted one untouched.
 - 2026-08-09: Kept in-repo agent scratch out of the Docker build context and out
   of version control. `.claude/` holds one worktree — an entire additional
   checkout of the repo — per in-flight agent, and under `COPY . .` all of it was

prompts/CODE_STYLEGUIDE_GO.md

@@ -1,6 +1,6 @@
 ---
 title: Code Styleguide — Go
-last_modified: 2026-08-09
+last_modified: 2026-08-10
 ---
 
 1.  Try to hard wrap long lines at 77 characters or less.
@@ -111,7 +111,11 @@ last_modified: 2026-08-09
 
 1.  For anything beyond a simple script or tool, or anything that is going to
     run in any sort of "production" anywhere, make sure it passes
-    `golangci-lint`.
+    `golangci-lint`. Run it with `make lint`, never by invoking the binary: the
+    linter always runs in a container, and `golangci-lint` is not installed on
+    the host by any repo. Invoked directly on a shared host it reads a result
+    cache keyed on file content rather than location, and a host-global lock, so
+    its answer may belong to another checkout entirely.
 
 1.  Write a `Dockerfile` for every repo, even if it only runs the tests and
     linting. `script/cibuild` and `script/docker` should always make sure that

prompts/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-08-09
+last_modified: 2026-08-10
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -42,6 +42,15 @@ with your task.
       `CHECK_EPOCH` rule in `REPO_POLICIES.md`. Without them the check layer is
       served from cache on an unchanged tree and the build reports a green it
       never ran.
+- [ ] **Every stage that runs checks sets `ENV LINT_IN_CONTAINER=1`** — the lint
+      stage and the build stage both. This is the item an existing repo most
+      often fails after adopting the containerised lint: without it
+      `script/lint` tries to build `Dockerfile.lint` from inside a build step,
+      where there is no daemon.
+- [ ] `Dockerfile.lint` exists and `script/lint` builds it when not already in a
+      container — see the containerised-lint rule in `REPO_POLICIES.md`. Base
+      image pinned by sha256 with a version/date comment, `ARG CHECK_EPOCH`
+      **after** the dependency layer with the guard below it.
 - [ ] `.dockerignore` excludes the repo's own host-built artifacts (compiled
       binaries, test binaries, coverage output), written root-anchored —
       `/myapp`, never `**/myapp`, which would also match `cmd/myapp/`. An
@@ -100,6 +109,24 @@ with your task.
       `script/install-precommit`, shimmed by `make hooks`) runs it
 - [ ] README has an **Entrypoints** section documenting the `script/`
       entrypoints and linking the standard
+- [ ] `script/lint` is the canonical detect-and-branch form, and no host
+      invocation anywhere in the repo can produce a lint **verdict** — grep for
+      the linter's own name across `script/`, the `Makefile` and CI config, not
+      just `script/lint`. A second path is likeliest here: a `make lint-fast`,
+      an older container-versus-host branch, or a CI step calling the binary
+      directly. **Expected hits that are not the defect**: `script/fmt`, and in
+      a repo whose formatter is also its linter, `script/fmt-check`. Everything
+      else the grep finds is a real second path and goes.
+- [ ] Detection is on `LINT_IN_CONTAINER` alone. Reject any `/.dockerenv` or
+      cgroup heuristic: absent in BuildKit `RUN` steps, present on hosts that
+      are themselves containers, and a false positive lints on the host.
+- [ ] `script/bootstrap` installs no golangci-lint. Delete the block, its
+      version and ref variables, and its call site. A JS repo's `yarn install`
+      stays — it brings a linter along with every other dependency, which is
+      fine as long as no verdict is taken from it.
+- [ ] The per-checkout lint state is gone: no `GOLANGCI_LINT_CACHE` or `TMPDIR`
+      exports, no `--allow-serial-runners`, and `.lint-cache/` removed from
+      `.gitignore` and `.dockerignore`.
 - [ ] `make check` does not modify any files in the repo
 - [ ] `make test` has a 30-second timeout
 - [ ] `make test` runs real tests, not a no-op (at minimum, import/compile
@@ -146,6 +173,9 @@ with your task.
 # Final
 
 - [ ] `make check` passes
-- [ ] `script/cibuild` succeeds (a bare `docker build .` fails closed by design,
+- [ ] `make lint` runs twice on an unchanged tree with the lint layer `DONE`
-      on the `CHECK_EPOCH` guard)
+      both times, never `CACHED` and never sub-second
+- [ ] `script/cibuild` succeeds (a bare `docker build .` or
+      `docker build -f Dockerfile.lint .` fails closed by design, on the
+      `CHECK_EPOCH` guard)
 - [ ] Commit and merge fixes before starting your actual task

prompts/NEW_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: New Repo Checklist
-last_modified: 2026-08-09
+last_modified: 2026-08-10
 ---
 
 Use this checklist when creating a new repository from scratch. Follow the steps
@@ -79,9 +79,21 @@ Template files can be fetched from:
       `CHECK_EPOCH` rule in `REPO_POLICIES.md`. Without them the check layer is
       served from cache on an unchanged tree and the build reports a green it
       never ran.
+    - Every stage that runs checks sets `ENV LINT_IN_CONTAINER=1`, so
+      `script/lint` runs the linter natively instead of trying to build
+      `Dockerfile.lint` where there is no daemon.
+    - Go repos: separate `lint` stage on the `golangci/golangci-lint` image,
+      with `COPY --from=lint /src/go.sum /dev/null` in the build stage to force
+      the ordering. Re-prove that ordering warm after adopting `CHECK_EPOCH`.
     - Server: also builds and runs the application
     - Non-server: brings up dev environment and runs `make check`
     - Image pinned by sha256 hash with version/date comment
+- [ ] `Dockerfile.lint` — the lint-only image `script/lint` builds when it is
+      not already inside a container. Sets `ENV LINT_IN_CONTAINER=1`; same
+      `ARG CHECK_EPOCH` + guard + expanded-value discipline as above, with the
+      `ARG` **after** the dependency layer so only the lint re-runs. Base image
+      pinned by sha256 with a version/date comment. Copy from
+      `REPO_POLICIES.md`.
 - [ ] Gitea Actions workflow at `.gitea/workflows/check.yml` that runs
       `script/cibuild` on push — reference
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitea/workflows/check.yml`
@@ -108,7 +120,15 @@ are thin shims calling them. Model scripts:
       then `install-precommit`, plus repo-specific init
 - [ ] `script/test` / `make test` — runs real tests, not a no-op (30-second
       timeout)
-- [ ] `script/lint` / `make lint` — runs linter
+- [ ] `script/lint` / `make lint` — runs the linter directly when
+      `LINT_IN_CONTAINER=1`, otherwise `epoch="$(date +%s%N)$$"` on its own line
+      then `docker build --build-arg CHECK_EPOCH="$epoch" -f Dockerfile.lint .`.
+      No lint verdict may come from a host invocation. Copy from
+      `REPO_POLICIES.md`. Detect on `LINT_IN_CONTAINER` only — never
+      `/.dockerenv`, which is absent in BuildKit `RUN` steps and present on
+      hosts that are themselves containers. Without the nonce this exits 0 on an
+      unchanged tree having linted nothing; without `-f Dockerfile.lint` it
+      builds the main image and lints nothing at all.
 - [ ] `script/fmt` / `make fmt` — formats code (writes)
 - [ ] `script/fmt-check` / `make fmt-check` — checks formatting (read-only)
 - [ ] `script/check` / `make check` — runs `test`, `lint`, `fmt-check`; must not
@@ -138,7 +158,10 @@ are thin shims calling them. Model scripts:
     export with no `.git` and on a repo with no commits — keep it, and do not
     collapse it into `|| echo unknown`, which makes it unreachable. See the
     `CHECK_EPOCH` and git-describe rules in `REPO_POLICIES.md` for why each
-    element is load-bearing. A bare `docker build .` fails closed by design.
+    element is load-bearing. A bare `docker build .` fails closed by design, and
+    so does a bare `docker build -f Dockerfile.lint .`. The image runs
+    `make check`, which includes lint, so `script/cibuild` needs no separate
+    lint step.
 
 - [ ] `script/precommit` — called by the pre-commit hook; runs `script/check`
 - [ ] `script/install-precommit` — installs the pre-commit hook that runs
@@ -150,7 +173,11 @@ are thin shims calling them. Model scripts:
 # 4. Verify
 
 - [ ] `make check` passes
+- [ ] `make lint` demonstrably runs the linter rather than returning a cached
+      build: run it twice on an unchanged tree and confirm the lint layer says
+      `DONE`, never `CACHED`, both times
 - [ ] `make docker` succeeds
+- [ ] `script/cibuild` succeeds and demonstrably executes
 - [ ] No secrets in repo
 - [ ] No mutable image/package references
 - [ ] No unnecessary files in repo root

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-08-09
+last_modified: 2026-08-10
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -94,32 +94,49 @@ style conventions are in separate documents:
   reading the Makefile.
 
 - Every repo should have a `Dockerfile`. All Dockerfiles must run `make check`
-  as a build step so the build fails if the branch is not green — which requires
+  as a build step so the build fails if the branch is not green — the one
-  `ARG CHECK_EPOCH` and its guard in every stage containing a check-running
+  exception being `Dockerfile.lint`, which runs `make lint` alone because that
-  `RUN`, per the `CHECK_EPOCH` rule below. Without them a Dockerfile satisfies
+  is its entire purpose — which requires `ARG CHECK_EPOCH` and its guard in
-  this criterion while its check layers are served from cache, so the build
+  every stage containing a check-running `RUN`, per the `CHECK_EPOCH` rule
-  cannot fail on a branch that is not green. For non-server repos, the
+  below. Without them a Dockerfile satisfies this criterion while its check
-  Dockerfile should bring up a development environment and run `make check`. For
+  layers are served from cache, so the build cannot fail on a branch that is not
-  server repos, `make check` should run as an early build stage before the final
+  green.
-  image is assembled. Dockerfiles install development prerequisites by running
+
-  `script/bootstrap` rather than duplicating installs inline; COPY `script/` and
+    **Every Dockerfile must also set `ENV LINT_IN_CONTAINER=1`**, above the
-  the dependency manifests (`package.json` + `yarn.lock`, `go.mod` + `go.sum`,
+    checks. `script/lint` builds `Dockerfile.lint` when it is not already in a
-  etc.) before running it so the bootstrap layer stays cached until dependencies
+    container; without the marker it would try that from inside a build step,
-  change.
+    where there is no daemon. See the containerised-lint rule below.
+
+    For non-server repos, the Dockerfile should bring up a development
+    environment and run `make check`. For server repos, `make check` should run
+    as an early build stage before the final image is assembled. Dockerfiles
+    install development prerequisites by running `script/bootstrap` rather than
+    duplicating installs inline; COPY `script/` and the dependency manifests
+    (`package.json` + `yarn.lock`, `go.mod` + `go.sum`, etc.) before running it
+    so the bootstrap layer stays cached until dependencies change.
 
 - **Every check-running `RUN` must be cache-busted with `CHECK_EPOCH`.** Docker
   invalidates a `COPY` layer only when the copied content changes, so on an
-  unchanged tree the `RUN make check` layer is served from cache, the suite
+  unchanged tree the check layer is served from cache, the suite never runs, and
-  never runs, and the build still exits 0. A sub-second `docker build` reporting
+  the build still exits 0. A sub-second `docker build` reporting success is a
-  success is a cache hit, not a result. The canonical form, in **every** stage
+  cache hit, not a result. This applies to **every** file that runs checks in a
-  containing a check-running `RUN`:
+  build step — `Dockerfile` and `Dockerfile.lint` alike; a `Dockerfile.lint`
+  without the cache-bust is a lint that never ran, reported as a pass. The
+  canonical form, in **every** stage containing a check-running `RUN`, placed
+  **after** the dependency-install layer so that layer stays cached:
 
     ```dockerfile
+    ENV LINT_IN_CONTAINER=1
     ARG CHECK_EPOCH
     RUN [ -n "$CHECK_EPOCH" ] || exit 1
     RUN echo "check epoch: ${CHECK_EPOCH}" && make check
     ```
 
+    `ENV LINT_IN_CONTAINER=1` belongs in every such stage too, and is the line
+    most often missed: without it `make check` reaches `script/lint`, which
+    tries to build `Dockerfile.lint` from inside a build step where there is no
+    daemon. See the containerised-lint rule below.
+
     and in both `script/cibuild` and `script/docker`:
 
     ```sh
@@ -132,6 +149,11 @@ style conventions are in separate documents:
         .
     ```
 
+    `script/lint` needs the same nonce but is **not** this command: it builds a
+    different file with `-f Dockerfile.lint` and passes no version. Copy its
+    form from the containerised-lint rule below, not this block — a
+    `docker build` with no `-f` builds the main image and lints nothing.
+
     The `VERSION` lines are there for a different reason, covered by the
     git-describe rule below; they are shown here so the two rules do not each
     document half a command. All four `CHECK_EPOCH` elements are load-bearing;
@@ -162,27 +184,189 @@ style conventions are in separate documents:
       concurrent invocations would collide.
 
     This invalidates the check layers and everything after them while leaving
-    `go mod download`, `script/bootstrap`, and the pinned toolchain install
+    `go mod download` and `script/bootstrap` cached, so it does not push against
-    cached, so it does not push against the five-minute Docker build ceiling.
+    the five-minute Docker build ceiling. Blanket `--no-cache` is not an
-    Blanket `--no-cache` also works but is wasteful and can blow that ceiling.
+    acceptable substitute: it also busts the dependency layer, so every run
+    reinstalls dependencies over the network instead of only the first and those
+    after a manifest change. Never reach for `docker builder prune` — the build
+    cache is shared with every other build on the host.
+
+- **Every lint run happens in a container.** `script/lint` runs the linter
+  directly when it is already inside one, and otherwise builds `Dockerfile.lint`
+  so that it is. Either way the linter never runs on a developer host, where its
+  answer is not trustworthy:
+    - **Confirmed false green.** golangci-lint keys cached results on file
+      **content, not location**, so a second checkout of the same commit serves
+      its findings. One implementer reported `0 issues` on a branch genuinely
+      red with a `goconst` finding. Own-clones-instead-of-worktrees does not
+      help; two clones are byte-identical exactly as two worktrees were.
+    - **False reds**: findings reported against other checkouts and against
+      worktrees already deleted; in one case 399 issues returned to a clean
+      clone that genuinely lints 0.
+    - **Lock contention indistinguishable from findings.** golangci-lint flocks
+      `$TMPDIR/golangci-lint.lock` (`pkg/commands/run.go`, `acquireFileLock()`),
+      host-global and independent of `GOLANGCI_LINT_CACHE`, 5-second timeout. It
+      prints `parallel golangci-lint is running`, analyzes nothing, exits
+      non-zero. Not fixed by per-cache isolation — measured.
+    - **Version skew**: a host linter differing from the pinned one, with the
+      container surfacing thirteen findings the host missed.
+
+    A container has its own cache, its own `TMPDIR` and a binary pinned by
+    digest, so none of it is reachable. This supersedes the per-checkout
+    `GOLANGCI_LINT_CACHE`/`TMPDIR` wrapper, which existed only to make a host
+    run trustworthy; delete it on adoption.
+
+    The canonical `script/lint`, whose executable lines are the same in every
+    repo apart from the native lint command:
+
+    ```sh
+    #!/bin/sh
+    # script/lint: run the linter. Inside a container, run it directly; on a
+    # host, build Dockerfile.lint so it runs in one anyway.
+    #
+    # LINT_IN_CONTAINER is set by this repo's Dockerfiles and is the ONLY
+    # accepted signal. Do not add a /.dockerenv fallback: it is absent inside
+    # BuildKit RUN steps and present on hosts that are themselves containers,
+    # so it both misses and false-positives — and a false positive silently
+    # restores host linting.
+    set -eu
+
+    ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+    main() {
+        cd "$ROOT"
+
+        if [ "${LINT_IN_CONTAINER:-}" = "1" ]; then
+            # config verify lives here, not in a Dockerfile, so every path
+            # that lints inherits it — the lint stage of the main image as
+            # well as Dockerfile.lint. Duplicating it into each Dockerfile
+            # is how one of them silently loses it.
+            golangci-lint config verify --config .golangci.yml
+            exec golangci-lint run --config .golangci.yml ./...
+        fi
+
+        # Own line, and `$$` because busybox `date` drops %N silently.
+        # Without a fresh nonce the lint layer is cached and this exits 0
+        # having linted nothing.
+        epoch="$(date +%s%N)$$"
+        docker build \
+            --build-arg CHECK_EPOCH="$epoch" \
+            -f Dockerfile.lint \
+            .
+    }
+
+    main "$@"
+    ```
+
+    and `Dockerfile.lint`, the standalone path for a developer host:
+
+    ```dockerfile
+    # Lint-only image, built by script/lint when not already in a container.
+    # golangci/golangci-lint:v2.12.2 (Debian-based), 2026-08-07
+    FROM golangci/golangci-lint@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240
+
+    WORKDIR /src
+    ENV LINT_IN_CONTAINER=1
+
+    COPY go.mod go.sum ./
+    RUN go mod download
+
+    COPY . .
+
+    # ARG after the dependency layer so only the lint re-runs.
+    ARG CHECK_EPOCH
+    RUN [ -n "$CHECK_EPOCH" ] || exit 1
+    RUN echo "lint epoch: ${CHECK_EPOCH}" && make lint
+    ```
+
+    Load-bearing properties:
+    - **Detection rests on `LINT_IN_CONTAINER=1` and nothing else.** Every
+      Dockerfile in the repo sets it; a host does not. The asymmetry is the
+      whole design: a **false negative** inside a container tries a nested
+      `docker build`, finds no daemon and fails loudly, while a **false
+      positive** on a host silently lints there — the exact defect this rule
+      exists to kill. So the signal must be one only our own images can produce.
+      `/.dockerenv` is not such a signal and must not be used, even as a
+      fallback: measured, it is **absent** inside BuildKit `RUN` steps and
+      **present** on any host that is itself a container, which is the common
+      case for CI runners and agent sandboxes. It fails in both directions, and
+      one of them is the dangerous one.
+    - **`CHECK_EPOCH`, not `--no-cache`.** `docker build -f Dockerfile.lint .`
+      on an unchanged tree returns a sub-second cached success having linted
+      nothing. The `ARG` goes **after** the dependency layer so only the lint
+      re-runs; `--no-cache` would also reinstall dependencies on every lint.
+    - **Non-Go repos get the same pattern around their own linter** — `eslint`,
+      `ruff`, `prettier`, `shellcheck`. Only the base image and the native lint
+      command change.
+    - **Keep `golangci-lint config verify`, put it in `script/lint`, and it
+      costs no network.** It goes in the native branch, not in a Dockerfile, so
+      the lint stage of the main image inherits it along with `Dockerfile.lint`;
+      putting it in one Dockerfile leaves the other path unverified. The two
+      commands catch disjoint classes, measured under the pinned v2.12.2: a
+      bogus top-level key and a bogus key under `linters.settings.lll` both pass
+      `golangci-lint run` with **exit 0 and `0 issues`** while `config verify`
+      exits 3 and names them; an invalid value type fails both; an unknown
+      linter name fails `run` and passes `config verify`. So `run` alone
+      silently ignores an unknown key — the mode where a threshold reads as
+      configured and is not applied. It needs no network: every case reproduced
+      byte-identically under `docker run --network none`, in a container where
+      `getent hosts golangci-lint.run` exits 2. The schema is embedded in the
+      pinned binary. Re-run that control when bumping the pin.
+    - **A failed `script/lint` that names no finding is not a lint result.** On
+      the host path `docker build` exits 1 both for findings and for a build
+      that never got there (daemon down, image unpullable, disk full). BuildKit
+      names the failing step; read it, fix the environment, re-run. Do not
+      record a verdict from a run that did not lint.
+
+    **Scope: this rule is about linters, and a formatter is not one.**
+    `script/fmt` writes your working tree, so it can only run on the host, and
+    `script/fmt-check` is its read-only twin. In a repo whose formatter **is**
+    its linter (prettier over markdown; this repo), `script/bootstrap` therefore
+    installs the linter on the host as an ordinary dependency and
+    `script/fmt-check` runs it there. That is accepted: the version is pinned in
+    `package.json` and installed into the repo's own `node_modules`, so there is
+    no shared content-keyed cache, no host-global lock and nothing to skew
+    against. What is forbidden is taking a **lint verdict** from it —
+    `script/lint` stays the only source of one. A repo auditing itself will see
+    those hits and should leave them; anything else the grep finds is a real
+    second path to the linter and goes.
+
+    **What a consuming repo does to adopt this**, in order:
+    1. Add `Dockerfile.lint`.
+    2. Replace `script/lint` with the form above, with its own native lint
+       command.
+    3. Add `ENV LINT_IN_CONTAINER=1` to **every** stage of every Dockerfile that
+       runs checks — the lint stage and the build stage both.
+    4. Delete any golangci-lint install from `script/bootstrap`, with its
+       version and ref variables and its call site. No lint verdict comes from
+       the host any more, so it can only reintroduce version skew. A JS repo's
+       `yarn install` stays.
+    5. Delete the per-checkout lint state: `GOLANGCI_LINT_CACHE` and `TMPDIR`
+       exports, `--allow-serial-runners`, the retry/VOID wrapper, and
+       `.lint-cache/` from both `.gitignore` and `.dockerignore`.
+    6. Verify by running `make lint` twice on an unchanged tree: the lint layer
+       must be `DONE` both times, never `CACHED`. Then plant a violation,
+       confirm it fails naming the finding, revert. A bare
+       `docker build -f Dockerfile.lint .` must fail on the guard.
+
+    `script/check`, `script/cibuild`, `script/docker` and the `Dockerfile` are
+    unchanged by this: `make check` still runs inside the image, and
+    `script/lint` there takes the native path.
 
 - **Dockerfiles must use a separate lint stage for fail-fast feedback.** Go
   repos use a multistage build where linting runs in an independent stage based
-  on the `golangci/golangci-lint` image (pinned by hash). This stage runs
+  on the `golangci/golangci-lint` image (pinned by hash), so lint failures
-  `make fmt-check` and `make lint` before the full build begins. The build stage
+  surface in seconds rather than after a full compile. The build stage declares
-  then declares an explicit dependency on the lint stage via
+  an explicit dependency on it via `COPY --from=lint /src/go.sum /dev/null`,
-  `COPY --from=lint /src/go.sum /dev/null`, which forces BuildKit to complete
+  which forces BuildKit — which runs stages in parallel by default — to finish
-  linting before proceeding to compilation and tests. This ensures lint failures
+  linting first. The canonical Go repo `Dockerfile`:
-  surface in seconds rather than minutes, without blocking on dependency
-  download or compilation in the build stage.
-
-    The standard pattern for a Go repo Dockerfile is:
 
     ```dockerfile
     # Lint stage — fast feedback on formatting and lint issues
     # golangci/golangci-lint:v2.x.x, YYYY-MM-DD
     FROM golangci/golangci-lint@sha256:... AS lint
     WORKDIR /src
+    ENV LINT_IN_CONTAINER=1
     COPY go.mod go.sum ./
     RUN go mod download
     COPY . .
@@ -195,6 +379,7 @@ style conventions are in separate documents:
     # golang:1.x-alpine, YYYY-MM-DD
     FROM golang@sha256:... AS builder
     WORKDIR /src
+    ENV LINT_IN_CONTAINER=1
 
     # Force BuildKit to run the lint stage before proceeding
     COPY --from=lint /src/go.sum /dev/null
@@ -221,50 +406,36 @@ style conventions are in separate documents:
     ```
 
     Key points:
-    - The lint stage uses the `golangci/golangci-lint` image directly (it
+    - The lint stage uses the `golangci/golangci-lint` image directly (it has
-      includes both Go and the linter), so there is no need to install the
+      both Go and the linter), so nothing needs installing. `make lint` there
-      linter separately.
+      runs `script/lint`, which sees `LINT_IN_CONTAINER=1` and invokes
-    - `COPY --from=lint /src/go.sum /dev/null` is a no-op file copy that creates
+      `golangci-lint` natively instead of building `Dockerfile.lint`. Without
-      a stage dependency. BuildKit runs stages in parallel by default; without
+      that `ENV` the stage would attempt a nested build and fail.
-      this line, the build stage would not wait for lint to finish and a lint
+    - `COPY --from=lint /src/go.sum /dev/null` is a no-op copy that exists only
-      failure might not fail the overall build.
+      to create the stage dependency; without it a lint failure might not fail
+      the overall build.
     - **Re-prove that ordering on a warm cache after adopting `CHECK_EPOCH`.**
-      The cache-bust turns this no-op `COPY` into a content-cache hit, so an
+      The cache-bust turns the no-op `COPY` into a content-cache hit, so an
-      ordering guarantee established on a cold cache does not automatically
+      ordering guarantee established cold does not automatically carry over. It
-      carry over; it has to be re-checked warm. This was re-proved in another
+      was re-proved in another org repo using the same trick and held, but that
-      repo in the org that uses the same file-dependency trick (there with a
+      result does not transfer by assumption — re-check it warm.
-      marker file in place of `go.sum`), and the ordering held. It has **not**
+    - If the project uses `//go:embed` referencing build artifacts, the lint
-      been verified in this repo, which is single-stage and has no lint stage to
+      stage must create placeholders so the directives resolve:
-      order against. Any repo relying on a file-dependency trick for stage
+      `RUN mkdir -p web/dist && touch web/dist/index.html`.
-      ordering should re-check it warm after adopting the bust rather than
+    - If linting needs CGO or system libraries (e.g. `vips-dev`), `apk add` them
-      assuming this result transfers.
+      in the lint stage.
-    - If the project uses `//go:embed` directives that reference build artifacts
+    - Tests run in the build stage, not the lint stage: they may need compiled
-      (e.g. a web frontend compiled in a separate stage), the lint stage must
-      create placeholder files so the embed directives resolve. Example:
-      `RUN mkdir -p web/dist && touch web/dist/index.html web/dist/style.css`.
-      The lint stage should not depend on the actual build output — it exists to
-      fail fast.
-    - If the project requires CGO or system libraries for linting (e.g.
-      `vips-dev`), install them in the lint stage with `apk add`.
-    - The build stage runs `make test` after compilation setup. Tests run in the
-      build stage, not the lint stage, because they may require compiled
       artifacts or heavier dependencies.
     - `ARG CHECK_EPOCH` appears in **both** stages, because `ARG` is
       stage-scoped: declaring it only in the lint stage leaves `make test`
-      frozen at the last cached result. In each stage the guard sits immediately
+      frozen at its last cached result. In each stage the guard sits immediately
-      below the `ARG` so a bare `docker build .` fails instead of reusing the
+      below the `ARG` and the value is expanded into the first check `RUN`.
-      empty cache key, and the value is expanded into the first check `RUN` so
+      Later `RUN`s in the same stage need no expansion; their parent layer is
-      the cache miss does not rely on BuildKit's unreferenced-`ARG` handling.
+      already busted.
-      Both of those lines reference `$CHECK_EPOCH`, so both are value-keyed:
+    - `ARG VERSION=dev` is declared in the build stage and supplied by
-      each stage is invalidated at two independent points. The later `RUN`s in
+      `script/docker` and `script/cibuild`. **No stage may call
-      the same stage need no expansion of their own: they are already
+      `git describe`**: `.dockerignore` excludes `.git`, so it yields an empty
-      invalidated by their busted parent layer.
+      version without failing. See the git-describe rule further down.
-    - `ARG VERSION=dev` is declared in the build stage, and its value is
-      supplied on the host by `script/docker` and `script/cibuild` via
-      `--build-arg VERSION=...`. The `dev` default is a placeholder for a local
-      build, not a source of truth. **No stage may call `git describe`**:
-      `.dockerignore` excludes `.git`, so it yields an empty version without
-      failing. See the git-describe rule further down.
 
 - Every repo should have a Gitea Actions workflow (`.gitea/workflows/`) that
   runs `script/cibuild` (which runs
@@ -274,9 +445,9 @@ style conventions are in separate documents:
   cache-bust described above. Without it, an unchanged tree serves the check
   layer from cache and the build reports a green it never earned. A bare
   `docker build .` fails closed by design, on the `[ -n "$CHECK_EPOCH" ]` guard;
-  always go through `script/cibuild` or `script/docker`. Never accept a
+  always go through `script/cibuild` or `script/docker`. Never accept a pass as
-  `script/cibuild` pass as evidence without confirming it ran: a sub-second wall
+  evidence without confirming it ran: a sub-second wall time, or `CACHED` on the
-  time, or `CACHED` on the check layer, means nothing was executed.
+  check layer, means nothing was executed.
 
 - Use platform-standard formatters: `black` for Python, `prettier` for
   JS/CSS/Markdown/HTML, `go fmt` for Go. Always use default configuration with
@@ -498,198 +669,111 @@ style conventions are in separate documents:
 - `.golangci.yml` is standardized and must _NEVER_ be modified by an agent, only
   manually by the user. Fetch from
   `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml`. The
-  canonical golangci-lint version is v2.12.2 (released 2026-05-06), installed
+  canonical golangci-lint version is v2.12.2 (released 2026-05-06), pinned as
-  commit-pinned via
+  the image digest in `Dockerfile.lint`
-  `go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5`.
+  (`golangci/golangci-lint@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240`,
+  which reports
+  `golangci-lint has version 2.12.2 built with go1.26.2 from c0d3ddc9`). That
+  digest is the only pin: golangci-lint is not installed on the host by any
+  repo. Bumping the version means changing that one digest.
 
-- **`script/bootstrap` in Go repos must install the pinned golangci-lint
+- **`script/bootstrap` must not install golangci-lint.** This supersedes the
-  whenever the installed version does not match the pin — not merely when the
+  pinned host install that used to be canonical here. `script/lint` never runs
-  binary is absent — and must then verify the install took effect by
+  it on the host — it either builds `Dockerfile.lint` or is already in a
-  re-resolving the binary through `PATH`.** The presence test
+  container that ships the binary — so a host install has no caller, and its
-  `if missing golangci-lint; then go install "$GOLANGCI_LINT_REF"; fi` is wrong:
+  only remaining effect is to put a second, independently-versioned linter where
-  it tests `PATH` presence and never version, so on any already-provisioned
+  somebody eventually runs it by hand and believes the result. Delete the block,
-  machine the pin is inert and a version bump is a no-op. Meanwhile the
+  its version and ref variables, and its call site.
-  Dockerfile installs unconditionally into a clean image, so CI and local
-  silently disagree about what the linter even is. Observed consequences: a
-  local `make check` green while `make docker` rejected the same commit with six
-  `goconst` findings, and a container linter surfacing thirteen findings the
-  host run missed. A stale host linter does not merely fail to prove the tree is
-  clean — it hides findings only the container can see. This is a deliberate
-  departure from the node handling described above, which uses whatever node is
-  installed: the linter version is the specific thing being held equal between
-  host and container, so for it, presence is not enough.
 
-    Comparing versions is necessary but **not sufficient**, because the obvious
+    This is not a ban on host dependency installs generally. A JS or docs repo's
-    fix also fails green. `go install` writes to `GOBIN` (or `GOPATH/bin`) while
+    `script/bootstrap` runs `yarn install`, which brings its linter along with
-    callers resolve `golangci-lint` through `PATH`. If a different binary
+    every other dependency; that is unavoidable and fine. The rule is about a
-    shadows it earlier in `PATH`, the install genuinely succeeds and changes
+    **dedicated** linter install, and about where a verdict may come from.
-    nothing any caller will ever see: bootstrap prints success and the next
-    `make lint` still runs the stale linter. That is worse than no fix, because
-    it converts a known-stale toolchain into one everyone believes is pinned.
-    The canonical form, placed in `script/bootstrap` after Go itself is present:
 
-    ```sh
+    **The version-enforcement principle it established still applies to any
-    # golangci-lint v2.12.2, 2026-05-06. GOLANGCI_LINT_VERSION must be exactly
+    other tool a repo pins and installs on the host**, and it is the part worth
-    # what `golangci-lint --version` prints for this ref; update both together.
+    keeping, because each of its four properties guards a failure that otherwise
-    GOLANGCI_LINT_VERSION="2.12.2"
+    reports success:
-    GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5"
+    - **Compare the installed version against the pin, never test presence.** A
-
+      `if missing <tool>; then install; fi` guard tests `PATH` presence and
-    # The version golangci-lint reports, resolved the way callers resolve it.
+      never version, so on any already-provisioned machine the pin is inert and
-    # Prints nothing when the binary is absent, exits non-zero, or prints
+      a version bump is a silent no-op. Compare the **whole** version token,
-    # something unparseable: all of those must read as "does not match".
+      exactly: a parser that stops at the first `-` reports `2.12.2` for a host
-    # The capture is the whole version token, not just its numeric prefix.
+      running `2.12.2-rc1` and skips the install — the original defect,
-    # Stopping at the first `-` would make 2.12.2-rc1 compare equal to 2.12.2
+      reintroduced through the comparison meant to fix it.
-    # and skip the install, which is the defect this whole rule exists to close.
-    # The trailing `|| true` is required, not tidiness. Under `set -o pipefail`
-    # a non-zero --version would otherwise propagate out of the pipeline and
-    # kill the script through `set -e` before the diagnostic below is printed.
-    golangci_lint_version() {
-        command -v golangci-lint >/dev/null 2>&1 || return 0
-        golangci-lint --version 2>/dev/null | head -n 1 |
-            sed -n 's/.*has version v\{0,1\}\([0-9][^ ]*\).*/\1/p' || true
-    }
-
-    ensure_golangci_lint() {
-        if [ "$(golangci_lint_version)" = "$GOLANGCI_LINT_VERSION" ]; then
-            echo "bootstrap: golangci-lint $GOLANGCI_LINT_VERSION already installed"
-            return 0
-        fi
-        echo "bootstrap: installing golangci-lint $GOLANGCI_LINT_VERSION"
-        go install "$GOLANGCI_LINT_REF"
-
-        # go install writes to GOBIN (or GOPATH/bin); callers resolve through
-        # PATH. Re-resolve through PATH and assert the install took effect.
-        # `hash -r` is load-bearing: without it a shell that already resolved
-        # a stale golangci-lint answers from its own lookup cache, and this
-        # check false-fails with the shadowing message below.
-        hash -r 2>/dev/null || true
-        gcl_got="$(golangci_lint_version)"
-        if [ "$gcl_got" = "$GOLANGCI_LINT_VERSION" ]; then
-            echo "bootstrap: golangci-lint $GOLANGCI_LINT_VERSION installed," \
-                "and PATH resolves it"
-            return 0
-        fi
-        gcl_bin="$(go env GOBIN)"
-        [ -n "$gcl_bin" ] || gcl_bin="$(go env GOPATH)/bin"
-        # Strip a trailing slash: GOBIN=/x/ would otherwise make the
-        # "$gcl_bin"/* test below miss and misreport shadowing.
-        while :; do
-            case "$gcl_bin" in
-                */) gcl_bin="${gcl_bin%/}" ;;
-                *) break ;;
-            esac
-        done
-        gcl_found="$(command -v golangci-lint 2>/dev/null || true)"
-        echo "bootstrap: installed golangci-lint $GOLANGCI_LINT_VERSION into" \
-            "$gcl_bin, but that is not what callers will get." >&2
-        case "$gcl_found" in
-            "")
-                echo "bootstrap: PATH resolves no golangci-lint at all." \
-                    "Add $gcl_bin to PATH, then re-run bootstrap." >&2
-                ;;
-            "$gcl_bin"/*)
-                echo "bootstrap: PATH resolves $gcl_found, inside that same" \
-                    "directory, reporting version ${gcl_got:-unparseable}." \
-                    "Nothing is shadowing it, so the install itself did not" \
-                    "produce the pinned version: check that" \
-                    "GOLANGCI_LINT_VERSION matches GOLANGCI_LINT_REF." >&2
-                ;;
-            *)
-                echo "bootstrap: PATH resolves $gcl_found instead, reporting" \
-                    "version ${gcl_got:-unparseable}. Remove that binary or" \
-                    "put $gcl_bin earlier in PATH, then re-run bootstrap." >&2
-                ;;
-        esac
-        exit 1
-    }
-
-    # The definitions above are inert on their own; the call site is part of
-    # the canonical form. In a script/bootstrap that follows the "define all
-    # functions, then call main" convention, this line belongs inside main()
-    # next to the other ensure_* steps.
-    ensure_golangci_lint
-    ```
-
-    Four properties are load-bearing; each guards a failure mode that otherwise
-    fails green:
-    - **Compare the installed version against the pin**, never test presence.
-      This is what makes a version bump propagate to machines that already have
-      some golangci-lint. Compare the **whole** version token, exactly: a parser
-      that stops at the first `-` reports `2.12.2` for a host running
-      `2.12.2-rc1`, which compares equal to a `2.12.2` pin and skips the install
-      — the original defect, reintroduced through the comparison meant to fix
-      it.
     - **After installing, re-resolve the binary the way callers resolve it** —
-      through `PATH`, not the path `go install` wrote to — and assert
+      through `PATH`, not the directory the installer wrote to — and assert the
-      `--version` reports the pin. When it does not, fail non-zero and name the
+      reported version is the pin. An installer that writes to `GOBIN` while a
-      path `command -v` actually found, the version it reports, and the
+      different binary shadows it earlier in `PATH` genuinely succeeds and
-      directory the install wrote to. That is a condition a human has to fix by
+      changes nothing any caller sees, which is worse than no fix: it converts a
-      hand, so bootstrap must not print success in it. Use `hash -r` first so
+      known-stale tool into one everyone believes is pinned. Run `hash -r` first
-      the shell does not answer from its own lookup cache. Diagnose the cause
+      so the shell does not answer from its own lookup cache, and when the
-      from the resolved path rather than asserting one: only a path **outside**
+      assertion fails, name the path `command -v` found, the version it reports,
-      the install directory is shadowing. When the resolved path is inside it,
+      and the directory the install wrote to. Diagnose from the resolved path
-      nothing is shadowing and telling the operator to delete that binary or
+      rather than asserting a cause: only a path **outside** the install
-      reorder `PATH` sends them after a fault that does not exist.
+      directory is shadowing.
     - **A mis-parse must fall through to reinstall, never to a false match.**
-      Absent binary, non-zero exit, empty output, and unrecognised output all
+      Absent binary, non-zero exit, empty output and unrecognised output should
-      yield an empty string, which compares unequal to the pin. The failure
+      all yield an empty string, which compares unequal to the pin. The failure
       direction is always a redundant install, never a skipped one.
-    - **Call it, and say so on success.** Two function definitions with no call
+    - **Call it, and say so on success.** A function defined and never called is
-      site are a silent no-op that reproduces the original defect exactly: exit
+      a silent no-op indistinguishable from success: exit 0, nothing installed,
-      0, nothing installed, no output, stale linter still resolved. A success
+      no output. Both success branches must print a line naming the version.
-      path that prints nothing is byte-identical to that no-op — same exit
+
-      status, same empty output — so both success branches must print a
+    Verifying such logic requires a negative control in an environment where a
-      confirmation naming the version. In a change about undetectable no-ops,
+    shadowing binary exists earlier in `PATH` than the install target — without
-      "it printed nothing and exited 0" must not be the healthy signal.
+    it the control passes against the naive compare-then-install form too and
+    proves nothing — plus a mis-parse control that feeds unparseable `--version`
+    output and confirms a reinstall. Run those controls against the block as a
+    consuming repo would adopt it: pasted into a `script/bootstrap`-shaped file
+    that is then executed, never by sourcing it and invoking the function
+    yourself. Driving the function directly tests something the artifact does
+    not do, and it is exactly how a missing call site passes every control while
+    the adopted snippet does nothing.
 
     Keep it POSIX sh: no bashisms, no arrays, no `[[`, no `grep -P`.
 
-    **On the hash-pinning rule.** `@c0d3ddc9cf3faa61a4e378e879ece580256d76e5` is
+- **Superseded: the per-checkout `GOLANGCI_LINT_CACHE`/`TMPDIR` wrapper for
-    a commit hash, not a server-mutable version tag, and the go command verifies
+  `script/lint`.** It existed only to make a host lint run trustworthy, and the
-    the fetched module against the checksum database — the mechanism the
+  containerised-lint rule above removes the host run. Delete the wrapper, the
-    hash-pinning rule at the top of this document already names as acceptable
+  `--allow-serial-runners` flag, and `.lint-cache/` from both `.gitignore` and
-    for Go modules. Note that `go install pkg@version` runs in module-aware mode
+  `.dockerignore`. Two of its conclusions outlive it: **`GOCACHE` does not need
-    ignoring the `go.mod` in the current directory or any parent, so no repo
+  isolating** (measured — content-addressed, no foreign paths in its entries, no
-    `go.sum` is consulted for this install; the checksum database is what
+  global lock), and **verifying lint plumbing requires paired controls** run
-    verifies it. The linter is a bootstrap prerequisite rather than part of any
+  against the artifact as a consuming repo would adopt it, since a control that
-    repo's module graph, which is why the canonical form installs it by
+  passes against the broken form proves nothing.
-    commit-pinned ref instead of declaring it in `go.mod`. Whether a `go.mod`
-    tool dependency — which would pin the hash in a committed, reviewable file
-    instead — should replace this is an open decision, tracked at
-    [prompts#37](https://git.eeqj.de/sneak/prompts/issues/37).
 
-    **Keep `GOLANGCI_LINT_VERSION` and the ref in sync.** The ref is a hash and
+- **Interim rule for reading a lint result produced on the host, in a repo that
-    carries no readable version, so the expected version is a separate string,
+  has not yet adopted the containerised lint above.** A lint run is **VOID**
-    and it must be exactly what `--version` prints for that ref — the comparison
+  unless both hold:
-    is an exact match on the whole version token. When the pinned commit carries
+    - the output contains no `parallel golangci-lint is running`, and
-    a release tag the go command resolves the hash to that tag, so the string is
+    - no reported file path begins with `../`, and none is an absolute path
-    simply the release number, `2.12.2` here. When it does not, the go command
+      outside the tree the run was launched from.
-    falls back to a pseudo-version and the binary reports something like
-    `2.12.3-0.20260506110758-c0d3ddc9cf3f`; that compares exactly like any other
-    string, so it works, but it cannot be known without building the binary once
-    and reading `--version` off it. Prefer pins on tagged releases for that
-    reason — the expected string is then derivable from the ref — not because
-    the comparison cannot handle the alternative.
 
-    Because the comparison covers the whole token, a pre-release is never
+    Do not record a verdict from a void run, and do not "fix" findings in files
-    confused with its release: a host carrying `2.12.2-rc1` against a `2.12.2`
+    the change does not touch — chasing phantom findings across untouched files
-    pin compares unequal and gets reinstalled. This matters more than it looks,
+    puts unrelated edits into a reviewed diff, which is more expensive than the
-    because a pre-release tag is still a tag, so a rule requiring merely that
+    wasted rework.
-    the pin be tagged would not catch it.
 
-    **Verifying a change to this logic requires a negative control run in an
+    The `../` clause is the one that actually bites, and it is why a filter
-    environment where a shadowing binary exists earlier in `PATH` than the
+    keyed on `/tmp` or on absolute prefixes is not enough: golangci-lint reports
-    install target.** Without that, the control passes against the naive
+    paths relative to its own resolved root rather than yours, and three of the
-    compare-then-install form as well and therefore proves nothing. Also check
+    org's reported sightings had relative paths and would have passed such a
-    the mis-parse direction by feeding it unparseable `--version` output and
+    filter. Both clauses are needed and neither alone is sufficient — one
-    confirming it reinstalls rather than reporting a match.
+    reproduction exited non-zero with the lock error and no foreign paths at
+    all, and another reported 34 well-formed findings, every one of them against
+    another checkout.
 
-    **Run those controls against the block as a consuming repo would adopt it**
+    **State the limit of these tests rather than treating them as a guarantee.**
-    — pasted into a `script/bootstrap`-shaped file that is then executed — not
+    They catch contamination that **names** foreign files. They cannot catch
-    by sourcing it and invoking the function yourself. Driving the function
+    contamination that **suppresses** findings through a poisoned entry for
-    directly tests something the artifact does not do, and it is exactly how a
+    colliding content, which has no wall-clock tell either — **no evidence of
-    missing call site passes every control while the adopted snippet does
+    that mode has been observed, and nobody should go chasing it**; the point is
-    nothing.
+    the reach of the tests, not a claim that the mode exists. They are a filter
+    for the loud mode, not a proof of soundness — which is the whole argument
+    for containerising the linter instead of documenting a discipline that
+    depends on every agent remembering to apply it. Adopt the rule above and
+    this one stops applying to the repo entirely.
 
 - When pinning images or packages by hash, add a comment above the reference
   with the version and date (YYYY-MM-DD).

script/cibuild

@@ -1,27 +1,22 @@
 #!/bin/sh
 # script/cibuild: run the CI build. The Dockerfile runs script/check, but
 # that only proves anything because CHECK_EPOCH is a fresh nonce on every
-# invocation: without it Docker serves the check layer from cache on an
+# invocation: without it Docker serves the check layer from cache and the
-# unchanged tree and the build exits 0 without running the suite.
+# build exits 0 without running the suite.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    # Assign on its own line: a failing command substitution inside an
+    # Both assignments on their own line: a failing command substitution
-    # argument does not trip `set -e`, which would silently degrade the
+    # inside an argument does not trip `set -e`, so the inline form
-    # nonce to an empty constant. `$$` is required because busybox `date`
+    # degrades silently to an empty constant. `$$` because busybox `date`
-    # drops %N without erroring.
+    # drops %N without erroring. VERSION is computed here because
+    # .dockerignore excludes .git, so `git describe` in a build stage
+    # yields an empty version without failing; the guard below is the
+    # single place the fallback is applied.
     epoch="$(date +%s%N)$$"
-    # VERSION must be computed here, on the host: .dockerignore excludes
-    # .git, so `git describe` cannot run in any build stage and fails
-    # quietly there rather than erroring. Same own-line discipline as the
-    # epoch. `|| true` keeps a failing describe from tripping `set -e`
-    # and leaves the value empty; the guard below is then the single
-    # place the fallback is applied, and it does fire — on an export with
-    # no .git, or a repo with no commits yet. `unknown` is visibly wrong
-    # in a binary in a way that an empty version is not.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build \

script/docker

@@ -11,19 +11,13 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    # Assign on its own line: a failing command substitution inside an
+    # Both assignments on their own line: a failing command substitution
-    # argument does not trip `set -e`, which would silently degrade the
+    # inside an argument does not trip `set -e`, so the inline form
-    # nonce to an empty constant. `$$` is required because busybox `date`
+    # degrades silently to an empty constant. `$$` because busybox `date`
-    # drops %N without erroring.
+    # drops %N without erroring. VERSION is computed here because
+    # .dockerignore excludes .git, so `git describe` in a build stage
+    # yields an empty version without failing.
     epoch="$(date +%s%N)$$"
-    # VERSION must be computed here, on the host: .dockerignore excludes
-    # .git, so `git describe` cannot run in any build stage and fails
-    # quietly there rather than erroring. Same own-line discipline as the
-    # epoch. `|| true` keeps a failing describe from tripping `set -e`
-    # and leaves the value empty; the guard below is then the single
-    # place the fallback is applied, and it does fire — on an export with
-    # no .git, or a repo with no commits yet. `unknown` is visibly wrong
-    # in a binary in a way that an empty version is not.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build \

script/lint

@@ -1,13 +1,34 @@
 #!/bin/sh
-# script/lint: run the linter.
+# script/lint: run the linter. Inside a container, run it directly;
+# on a host, build Dockerfile.lint so it runs in one anyway. The linter
+# is never run on a developer host, where a shared result cache, a
+# host-global lock and a stale toolchain make its answer untrustworthy.
+#
+# LINT_IN_CONTAINER is set by this repo's Dockerfiles and is the ONLY
+# accepted signal. Do not add a /.dockerenv fallback: it is absent
+# inside BuildKit RUN steps and present on hosts that are themselves
+# containers, so it both misses and false-positives — and a false
+# positive silently restores host linting.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    echo "Linting markdown files..."
+
-    yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
+    if [ "${LINT_IN_CONTAINER:-}" = "1" ]; then
+        exec yarn run prettier --check '**/*.md' \
+            --tab-width 4 --prose-wrap always
+    fi
+
+    # Own line, and `$$` because busybox `date` drops %N silently.
+    # Without a fresh nonce the lint layer is cached and this exits 0
+    # having linted nothing.
+    epoch="$(date +%s%N)$$"
+    docker build \
+        --build-arg CHECK_EPOCH="$epoch" \
+        -f Dockerfile.lint \
+        .
 }
 
 main "$@"