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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.