script/cibuild was a plain `docker build .` and the Dockerfile does
`COPY . .` followed by `RUN make check`, so on an unchanged tree Docker
served the check layer from cache: the suite never ran and the build
still exited 0. Measured here before the change, a second run on a
byte-identical tree returned in 0.286s with `RUN make check` CACHED.
script/cibuild and script/docker now pass --no-cache. The canonical text
asserting that a bare `docker build .` proves the checks ran was wrong in
REPO_POLICIES.md, both checklists and the Go styleguide, and is corrected
in all of them.
Model: opus-5
