script/lint and script/test build their Dockerfile phase with
--output type=cacheonly in place of a tag. The phase still runs
uncached and a failing step still fails the build, but no image is
written: nothing used those images, and writing one out took about 16
seconds of a Go repository's test build. script/cibuild and
script/docker keep their tags. REPO_POLICIES.md, both checklists and
the README now say the gate builds write no image.
Model: opus-5-5
sneak