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.

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