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

The linter is no longer installed on the host and no longer invoked
there. script/lint is now `docker build -f Dockerfile.lint .` and
nothing else, with the linter running as a build step, so a successful
build of that file is a clean lint — and it works unchanged where the
docker daemon is remote and bind mounts are impossible.

That removes three host-only failure mechanisms rather than mitigating
them: the result cache keyed on file content rather than location, which
produced a confirmed false green and a string of findings reported
against other checkouts; the host-global $TMPDIR/golangci-lint.lock,
which fails a run with `parallel golangci-lint is running` in a way no
caller can distinguish from findings; and host/container version skew,
which hid thirteen findings on one repo. A container per run has its own
cache, its own lock and a binary pinned by digest.

Resolving the recursion this creates. script/lint is a docker build, so
a Dockerfile that runs `make check` would nest a build inside a build
step where there is no daemon. Fixed by direction, not detection: the
main Dockerfile runs script/test and script/fmt-check individually, with
a comment saying why `make check` must not come back, and script/cibuild
runs script/lint first for fail-fast feedback. script/check still runs
all three, so developers and the pre-commit hook are unaffected.

Dockerfile.lint carries the same CHECK_EPOCH guard as the main image,
with the ARG placed below the dependency layer so only the lint steps
re-run. Blanket --no-cache was rejected: it re-runs the dependency
install on every lint and makes linting network-dependent.

golangci-lint config verify is kept, on measurement rather than
preference. Under the pinned v2.12.2, a bogus top-level key and a bogus
key nested 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. The two catch
disjoint classes, and `run` alone silently ignores the class where a
threshold reads as configured and is not applied. The concern that
config verify fetches its JSON schema over live HTTPS does not hold for
this version: 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.

Two canonical forms are superseded and deleted rather than left standing
beside the new one, because consuming repos read these documents
literally and two contradictory canonical script/lint forms is worse
than either. The script/bootstrap golangci-lint install landed for
#28 is removed: nothing invokes
a host linter now, so it can only reintroduce the skew it was written to
close. Its version-enforcement principle — compare version not presence,
re-resolve through PATH after installing, let a mis-parse fall through
to reinstall, and call it — stays documented for any other pinned host
tool. The per-checkout GOLANGCI_LINT_CACHE/TMPDIR wrapper is removed
with it; its entire subject was making a host run trustworthy. Adopting
repos delete .lint-cache/ from .gitignore and .dockerignore too. The Go
multistage lint stage and its COPY --from=lint ordering trick go the
same way: that stage ran `make lint`, which is now a docker build.

Corrected everywhere the claim that a successful docker build implies
lint passed — REPO_POLICIES.md, both repo checklists, the Go styleguide
and the README. The guarantee now belongs to script/cibuild, which runs
both container builds; a bare `docker build .` never lints at all.

Verified in this repo, not only documented: two consecutive script/lint
runs on a byte-identical tree both executed prettier (4.556s and 3.738s,
lint layers DONE with a fresh epoch printed, dependency layers CACHED as
intended); a planted violation failed the build naming the file, and
reverting it went green; a bare `docker build -f Dockerfile.lint .`
failed on the guard; make check, script/docker and script/cibuild all
green with the check layers demonstrably executing; and the main image
build completed without attempting a nested build.

Rework, from independent review of this commit. The canonical text is
the deliverable here, so a false sentence is a fleet-wide defect: the
Dockerfile rule still said the build "fails if the branch is not green",
which stopped being true when lint left that image, and the earlier
sweep grepped one phrasing rather than the claim. Re-swept on the claim
itself — green/red-branch wording, build-fails wording, entailment verbs
near build/lint/check, and "linted" asserted as covered — across
prompts/, README.md, TODO.md, both Dockerfiles and every script.

Two absolutes are narrowed to what is actually true, because seventeen
repos adopt this literally. The rule is that no lint VERDICT may come
from a host invocation, not that the binary never exists on the host: a
JS repo's `yarn install` puts its linter in node_modules on the host
unavoidably, and in a repo whose formatter is its linter — this one —
`script/fmt-check` runs the same command that Dockerfile.lint runs. That
gap is now stated with its bound (the version is pinned in the repo's
own node_modules, so no shared cache, no host lock, nothing to skew) and
the audit grep keeps its reach, gaining a note on which two hits are
expected rather than being weakened.

script/lint conflates "found issues" with "could not run": docker build
exits 1 for both. The exit-75 VOID machinery is deliberately not
restored, and the reasoning is now recorded where a reader looking for
it lands. The dangerous direction is already closed, since a build that
cannot run fails closed and can never read as clean; BuildKit already
names the failing step, where the old lock error went to stderr while
findings went to stdout and was easy to lose; and the failure is not
transient, so the retry that justified the old machinery would be wrong
here. Rebuilding the distinction would mean per-invocation capture files
and traps again plus matching on BuildKit's message format, which is not
a stable interface, and a mis-match in the "treat as infrastructure"
direction would be the false green this rule exists to prevent. What
survives is binding as a reading rule: a run that did not reach the lint
step is not a verdict.

Also corrected: script/lint was listed above a CHECK_EPOCH snippet that
does a bare `docker build .` with no -f, which would have built the main
image and linted nothing; the canonical Go Dockerfile template used
`make fmt-check` / `make test` where every prose rule in the same
document says script/, one Makefile edit away from re-entering the
recursion; README omitted the mandatory VERSION build arg; script/docker
did not say lint had left its image, a comment that propagates
fleet-wide; the "byte-identical across repos" claim for script/lint is
narrowed to its executable lines; and "--no-cache makes linting
network-dependent" is softened to the measured comparative claim.

TODO.md

@@ -21,6 +21,48 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-08-10: Moved every lint run into a container, on the owner's ruling, and
+  made this repo do it rather than merely document it. `script/lint` is now
+  `docker build -f Dockerfile.lint .` and nothing else; the linter is never
+  installed on the host and never invoked there, so a run cannot inherit another
+  checkout's content-keyed result cache, the host-global
+  `$TMPDIR/golangci-lint.lock`, or a host toolchain that differs from the pinned
+  one — the three mechanisms behind a confirmed false green, a string of
+  findings reported against other agents' checkouts, and a container that saw
+  thirteen findings the host missed. Linting runs as a build step, so a
+  successful build is a clean lint, which also works where the docker daemon is
+  remote and bind mounts are impossible. The recursion this creates is resolved
+  by direction rather than by detection: the main `Dockerfile` runs the
+  individual non-lint checks instead of `make check`, and `script/cibuild` runs
+  `script/lint` first, so no build ever nests a build. `Dockerfile.lint` carries
+  the same `CHECK_EPOCH` guard as the main image, with the `ARG` below the
+  dependency layer so only the lint steps re-run — blanket `--no-cache` was
+  rejected because it makes every lint reinstall its dependencies over the
+  network. Two canonical forms were superseded rather than left standing beside
+  the new one, since consuming repos read this document literally: the
+  `script/bootstrap` golangci-lint install (nothing runs a host linter now, so
+  it can only reintroduce skew; the version-enforcement principle stays
+  documented for other pinned host tools) and the per-checkout
+  cache/lock/`.lint-cache` wrapper (its whole subject was making a host run
+  trustworthy). The Go multistage lint stage goes with them: it ran `make lint`,
+  which is now a docker build. `golangci-lint config verify` was kept on
+  measurement, not preference — a bogus config key passes `golangci-lint run`
+  with `0 issues` and fails `config verify`, and every case reproduced
+  byte-identically under `docker run --network none`, so the schema is embedded
+  in the pinned binary and the line costs no network. Two positions are stated
+  rather than left as gaps, because canon that omits them gets re-derived
+  wrongly: `docker build` returns 1 both for findings and for a build that never
+  reached the lint step, and the exit-75 VOID machinery is deliberately not
+  restored — the dangerous direction is closed since an unrunnable lint fails
+  closed, BuildKit already names the failing step, and the failure is not
+  transient, so what survives is the reading rule that a run which did not lint
+  is not a verdict. And the rule is about linters: `script/fmt` and
+  `script/fmt-check` run on the host by necessity, which in a repo whose
+  formatter is its linter — this one — means that exact command does run there,
+  recorded as a known and bounded gap rather than papered over. Verified with
+  two consecutive runs on an unchanged tree both executing the linter, a planted
+  violation caught and reverted, the bare-build guard firing, and the main image
+  building without attempting a nested build.
 - 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