A bare `docker build .` served the Dockerfile's check steps from the
layer cache on an unchanged tree and exited 0 without running them.
script/cibuild now computes the version on its own line and runs the
same build command as script/docker and the shared policy, --no-cache
included, so every CI build runs the checks. README.md describes
script/cibuild accordingly.
Model: opus-5-5
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