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.

Because the guard references $CHECK_EPOCH it is itself value-keyed:
BuildKit renders the epoch into that layer's description and re-runs the
layer when the value changes. Each stage therefore has two independent
invalidation points, the guard and the expansion, and the guard always
precedes the check RUN. Both are kept and the prose now records this;
the expansion remains defence in depth and is what puts the epoch in the
build log.

The false guarantee was org-canonical text in more than one document, so
it is corrected everywhere it appeared rather than only where the issue
first found it. REPO_POLICIES.md carried it 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. CODE_STYLEGUIDE_GO.md restated the guarantee for the bare command
this change makes fail closed. NEW_REPO_CHECKLIST.md specified the
pre-fix script/cibuild verbatim, so every new repo would have been born
with the false green, and EXISTING_REPO_CHECKLIST.md ended on a
`docker build` acceptance item that the guard makes unsatisfiable by
design — an agent working that checklist would have been led to delete
the guard to tick the last box. Both checklists' Dockerfile criteria were
also satisfiable by a Dockerfile whose check layers are still frozen, and
now require the ARG and guard in every check-running stage.

README.md

@@ -123,8 +123,9 @@ alpine. We provide:
 - `script/check` — run all checks: `test`, `lint`, `fmt-check` (our own
   extension)
 - `script/docker` — build the Docker image, tagged via `script/projectname`
-  (byte-identical across repos)
+  (byte-identical across repos); passes the same `CHECK_EPOCH` nonce as
-- `script/cibuild` — cd to the repo root and
+  `script/cibuild`
+- `script/cibuild` — cd to the repo root, assign `epoch="$(date +%s%N)$$"`, then
   `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