Merge pull request 'ci: build the image once instead of twice' (#7) from ci/one-build-not-two into main

Reviewed-on: #7

.gitea/workflows/build-and-publish.yaml

@@ -45,7 +45,8 @@ jobs:
 
       # The suite runs inside the image, against the interpreter that ships,
       # rather than against whatever the runner happens to provide. A failing
-      # test fails the build. Layers are shared with the push build below.
+      # test fails the build. The runtime stage below is built from the same
+      # daemon afterwards, so its layers are already in cache.
       - name: Run the test suite inside the image
         run: docker build --target test -t music-curator:test .
 
@@ -124,9 +125,6 @@ jobs:
           echo "release=${release}" >> "$GITHUB_OUTPUT"
           echo "Computed bump=${bump}, release=${release}, base=${base}"
 
-      - name: Set up Buildx
-        uses: docker/setup-buildx-action@d7f5e7f509e45cec5c76c4d5afdd7de93d0b3df5 # v4
-
       - name: Log in to the Gitea container registry
         if: github.event_name != 'pull_request'
         uses: docker/login-action@650006c6eb7dba73a995cc03b0b2d7f5ca915bee # v4
@@ -135,21 +133,33 @@ jobs:
           username: ${{ github.repository_owner }}
           password: ${{ secrets.PACKAGES_TOKEN }}
 
-      - name: Build and push
-        uses: docker/build-push-action@f9f3042f7e2789586610d6e8b85c8f03e5195baf # v7
-        with:
-          context: .
-          # Without this the last stage in the Dockerfile -- the test stage --
-          # would be what gets published.
-          target: runtime
-          # The NAS is the only host this runs on. Building arm64 as well would
-          # mean emulating it under QEMU for no consumer.
-          platforms: linux/amd64
-          push: ${{ github.event_name != 'pull_request' }}
-          tags: ${{ steps.version.outputs.tags }}
-          labels: |
-            org.opencontainers.image.source=${{ github.server_url }}/${{ github.repository }}
-            org.opencontainers.image.revision=${{ github.sha }}
+      # Plain `docker build` rather than buildx. buildx boots its own buildkit
+      # in a container with a cache of its own, so it shared nothing with the
+      # test build above and rebuilt the image from the base image up -- two
+      # full builds per run. It earns that cost when building for several
+      # platforms; this only ever targets the amd64 NAS, so it does not.
+      #
+      # `--target runtime` is a strict prefix of the test stage, so every layer
+      # is already in the daemon's cache and this resolves in seconds.
+      - name: Build the runtime image
+        run: |
+          set -euo pipefail
+          tags=()
+          while IFS= read -r tag; do
+            [ -n "$tag" ] && tags+=(-t "$tag")
+          done <<< "${{ steps.version.outputs.tags }}"
+          docker build --target runtime \
+            --label "org.opencontainers.image.source=${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}" \
+            --label "org.opencontainers.image.revision=${GITHUB_SHA}" \
+            "${tags[@]}" .
+
+      - name: Push
+        if: github.event_name != 'pull_request'
+        run: |
+          set -euo pipefail
+          while IFS= read -r tag; do
+            [ -n "$tag" ] && docker push "$tag"
+          done <<< "${{ steps.version.outputs.tags }}"
 
       # Record the release: write the computed version into pyproject.toml, then
       # commit and tag it, so the packaging metadata always matches the release