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Emma Thorpe ef59e52bca
Build and publish container / build (pull_request) Successful in 4m4s
ci: build the image once instead of twice
Runs take fifteen to eighteen minutes, and the log shows why: the image is
built twice, in full.

The test stage is built by the runner's docker daemon. The runtime stage was
then built by docker/build-push-action, which runs under a buildx builder that
setup-buildx-action creates in its own container with its own cache. The two
share nothing, so the second build spent seventy-six seconds booting buildkit
and then installed ffmpeg and the package all over again -- around a hundred
and ten seconds for the apk and another hundred for pip, neither of which
produced anything the first build had not already made. The comment above the
test step claimed those layers were shared, which is what made this look
reasonable.

buildx earns that overhead when producing several architectures. This produces
linux/amd64 only, by an explicit decision recorded in the workflow, so it earns
nothing here. Use plain docker build against the same daemon that ran the
tests, and push with docker push. The runtime stage is a strict prefix of the
test stage, so every layer is a cache hit: measured at 1.3 seconds locally.

Identical to the change made in music-curator, whose workflow this one was
copied from.
2026-08-24 17:29:54 +01:00
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