Bust the Docker check-layer cache with a per-invocation CHECK_EPOCH (closes #26)

script/cibuild was a plain `docker build .`, and the Dockerfile does
`COPY . .` followed by `RUN make check`. Docker invalidates a COPY layer
only when the copied content changes, so on an unchanged tree the check
layer was served from cache, the suite never ran, and the build still
exited 0. Measured here: run 1 took 18.5s and ran the suite; run 2 on a
byte-identical tree took 0.286s with `RUN make check` CACHED.

script/cibuild and script/docker now assign a per-invocation nonce on its
own line and pass it as --build-arg CHECK_EPOCH. The Dockerfile declares
ARG CHECK_EPOCH, guards it with `[ -n "$CHECK_EPOCH" ] || exit 1`, and
expands it into the check command. Post-fix, two consecutive runs both
execute make check (17.4s / 8.1s) with `RUN script/bootstrap` still
CACHED, so dependency layers are untouched and the build ceiling is not
at risk.

The guard is what makes a bare `docker build .` — the command
REPO_POLICIES named verbatim — fail closed rather than reuse the empty
and therefore stable cache key; verified failing in 0.455s. Holding the
epoch constant restores the false green (run 2 fully CACHED), which pins
the varying value as the operative mechanism rather than a coincidence.

REPO_POLICIES.md carried the false guarantee as org-canonical text in
two places, and its Go multistage template had check steps in two
stages; ARG is stage-scoped, so both stages get the treatment or the
fleet inherits the half-fixed shape.

Dockerfile

@@ -12,4 +12,15 @@ COPY package.json yarn.lock ./
 RUN script/bootstrap
 
 COPY . .
-RUN make check
+
+# CHECK_EPOCH is a per-invocation nonce supplied by script/cibuild and
+# 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
+# makes a bare `docker build .` fail loudly instead of silently reusing
+# the empty (and therefore stable) cache key. Expand the value into the
+# command so the cache miss does not depend on BuildKit's handling of an
+# unreferenced ARG.
+ARG CHECK_EPOCH
+RUN [ -n "$CHECK_EPOCH" ] || exit 1
+RUN echo "check epoch: ${CHECK_EPOCH}" && make check

README.md

@@ -124,8 +124,11 @@ alpine. We provide:
   extension)
 - `script/docker` — build the Docker image, tagged via `script/projectname`
   (byte-identical across repos)
-- `script/cibuild` — cd to the repo root and `docker build .` (what CI runs; the
-  image build runs `script/check`)
+- `script/cibuild` — cd to the repo root and
+  `docker build --build-arg CHECK_EPOCH="$epoch" .` (what CI runs; the image
+  build runs `script/check`, and the per-invocation `CHECK_EPOCH` nonce is what
+  stops Docker serving that check from cache on an unchanged tree — a bare
+  `docker build .` fails closed on purpose)
 - `script/precommit` — run by the git pre-commit hook (our own extension); calls
   `script/check`
 - `script/install-precommit` — installs the git pre-commit hook (our own

TODO.md

@@ -21,6 +21,12 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-08-09: Fixed the false green in the canonical CI gate: `script/cibuild`
+  and `script/docker` now pass a per-invocation `CHECK_EPOCH` nonce, and the
+  `Dockerfile` (plus the Go multistage template in REPO_POLICIES.md, in both its
+  lint and builder stages) declares `ARG CHECK_EPOCH` with a guard that makes a
+  bare `docker build .` fail closed. Corrected the org-canonical text that
+  asserted a successful build implies all checks pass.
 - 2026-08-07: Set the canonical `.golangci.yml` to the org-standard v2-schema
   config already deployed byte-identical across the org's Go repos (settings
   under `linters.settings` so thresholds like lll/funlen/cyclop/dupl actually

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-08-07
+last_modified: 2026-08-09
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -60,17 +60,19 @@ style conventions are in separate documents:
   prerequisite since nvm requires bash. yarn is then pinned via
   `corepack prepare yarn@<version> --activate`. Never install "latest" or "lts";
   always exact versions. `script/cibuild` runs the CI build: it changes to the
-  repo root and runs `docker build .`; the Gitea workflow calls it. Four further
-  scripts are our own extensions to the standard: `script/check` runs
-  `script/test`, `script/lint`, and `script/fmt-check`; `script/precommit` is
-  what the git pre-commit hook runs, and it calls `script/check`;
-  `script/install-precommit` installs the git pre-commit hook (the `make hooks`
-  target shims to it); and `script/projectname` (literally that filename) simply
-  outputs the project's name. Scripts that need the name call
-  `script/projectname` — e.g. `script/docker` assembles its image tag from it —
-  so those scripts stay byte-identical across all repos. Repo-type-specific
-  pre-commit extras (e.g. `go mod tidy` verification in Go repos) belong in
-  `script/precommit`, not in the hook itself. Model scripts are at
+  repo root and runs `docker build --build-arg CHECK_EPOCH="$epoch" .`, where
+  `epoch` is a per-invocation nonce (see the `CHECK_EPOCH` rule below); the
+  Gitea workflow calls it. Four further scripts are our own extensions to the
+  standard: `script/check` runs `script/test`, `script/lint`, and
+  `script/fmt-check`; `script/precommit` is what the git pre-commit hook runs,
+  and it calls `script/check`; `script/install-precommit` installs the git
+  pre-commit hook (the `make hooks` target shims to it); and
+  `script/projectname` (literally that filename) simply outputs the project's
+  name. Scripts that need the name call `script/projectname` — e.g.
+  `script/docker` assembles its image tag from it — so those scripts stay
+  byte-identical across all repos. Repo-type-specific pre-commit extras (e.g.
+  `go mod tidy` verification in Go repos) belong in `script/precommit`, not in
+  the hook itself. Model scripts are at
   `https://git.eeqj.de/sneak/prompts/raw/branch/main/script/<name>`. The README
   must document the provided scripts in an **Entrypoints** section (see the
   README requirements below).
@@ -99,6 +101,51 @@ style conventions are in separate documents:
   `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
+  never runs, and the build still exits 0. A sub-second `docker build` reporting
+  success is a cache hit, not a result. The canonical form, in **every** stage
+  containing a check-running `RUN`:
+
+    ```dockerfile
+    ARG CHECK_EPOCH
+    RUN [ -n "$CHECK_EPOCH" ] || exit 1
+    RUN echo "check epoch: ${CHECK_EPOCH}" && make check
+    ```
+
+    and in both `script/cibuild` and `script/docker`:
+
+    ```sh
+    epoch="$(date +%s%N)$$"
+    docker build --build-arg CHECK_EPOCH="$epoch" .
+    ```
+
+    All four elements are load-bearing; none is optional, and each guards a
+    failure mode that otherwise fails green:
+    - `ARG` is stage-scoped, so a single declaration leaves the other check
+      stages frozen while the fix reviews as complete. Declare it in every stage
+      that runs checks, immediately above the first such `RUN`.
+    - Expand the value into the command. This makes the cache miss contractual
+      rather than dependent on BuildKit's handling of an unreferenced `ARG`, and
+      it puts the epoch in the build log.
+    - The `[ -n ... ]` guard is required: an unset `ARG` is empty, and empty is
+      a stable cache key, so without it a bare `docker build .` still produces
+      the false green. Failed steps are never cached, so the guard fails on
+      every such invocation, loudly. A bare `docker build .` failing is by
+      design.
+    - Assign `epoch=` on its own line, never inline in the `--build-arg`
+      argument: a failing command substitution inside an argument does not trip
+      `set -e`, so the inline form silently degrades to an empty constant. The
+      `$$` suffix is required because busybox `date` drops `%N` and exits 0, so
+      on an alpine host the epoch would degrade to second granularity and
+      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
+    cached, so it does not push against the five-minute Docker build ceiling.
+    Blanket `--no-cache` also works but is wasteful and can blow that ceiling.
+
 - **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
@@ -119,7 +166,9 @@ style conventions are in separate documents:
     COPY go.mod go.sum ./
     RUN go mod download
     COPY . .
-    RUN make fmt-check
+    ARG CHECK_EPOCH
+    RUN [ -n "$CHECK_EPOCH" ] || exit 1
+    RUN echo "check epoch: ${CHECK_EPOCH}" && make fmt-check
     RUN make lint
 
     # Build stage
@@ -133,7 +182,9 @@ style conventions are in separate documents:
     COPY go.mod go.sum ./
     RUN go mod download
     COPY . .
-    RUN make test
+    ARG CHECK_EPOCH
+    RUN [ -n "$CHECK_EPOCH" ] || exit 1
+    RUN echo "check epoch: ${CHECK_EPOCH}" && make test
 
     ARG VERSION=dev
     RUN CGO_ENABLED=0 go build -trimpath \
@@ -165,11 +216,26 @@ style conventions are in separate documents:
     - 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
+      below the `ARG` so a bare `docker build .` fails instead of reusing the
+      empty cache key, and the value is expanded into the first check `RUN` so
+      the cache miss does not rely on BuildKit's unreferenced-`ARG` handling.
+      The later `RUN`s in the same stage need no expansion of their own: they
+      are already invalidated by their busted parent layer.
 
 - Every repo should have a Gitea Actions workflow (`.gitea/workflows/`) that
-  runs `script/cibuild` (which runs `docker build .`) on push. Since the
-  Dockerfile already runs `make check`, a successful build implies all checks
-  pass.
+  runs `script/cibuild` (which runs
+  `docker build --build-arg CHECK_EPOCH="$epoch" .`) on push. The Dockerfile
+  runs `make check`, so a successful build implies all checks pass — but that
+  implication holds **only** because of the `CHECK_EPOCH` cache-bust described
+  above. Without it, an unchanged tree serves the check layer from cache and the
+  build reports a green it never earned. A bare `docker build .` fails closed by
+  design, on the `[ -n "$CHECK_EPOCH" ]` guard; always go through
+  `script/cibuild` or `script/docker`. Never accept a `script/cibuild` pass as
+  evidence without confirming it ran: a sub-second wall time, or `CACHED` on the
+  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

script/cibuild

@@ -1,13 +1,20 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs script/check, so
-# a successful build implies all checks pass.
+# 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
+# unchanged tree and the build exits 0 without running the suite.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build .
+    # Assign on its own line: a failing command substitution inside an
+    # argument does not trip `set -e`, which would silently degrade the
+    # nonce to an empty constant. `$$` is required because busybox `date`
+    # drops %N without erroring.
+    epoch="$(date +%s%N)$$"
+    docker build --build-arg CHECK_EPOCH="$epoch" .
 }
 
 main "$@"

script/docker

@@ -1,6 +1,9 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
-# Identical in all repos; the tag comes from script/projectname.
+# Identical in all repos; the tag comes from script/projectname. The
+# Dockerfile's checks only actually run because CHECK_EPOCH is a fresh
+# nonce on every invocation; without it a warm cache turns this into a
+# green that proves nothing.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -8,7 +11,13 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build -t "$("$SCRIPT_DIR/projectname")" .
+    # Assign on its own line: a failing command substitution inside an
+    # argument does not trip `set -e`, which would silently degrade the
+    # nonce to an empty constant. `$$` is required because busybox `date`
+    # drops %N without erroring.
+    epoch="$(date +%s%N)$$"
+    docker build --build-arg CHECK_EPOCH="$epoch" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"