Track the CI image hash with a label instead of a tag

Pushing per-hash tags accumulates multi-GB versions in the registry,
and package cleanup rules can't distinguish the hash tags that the
latest-{arch} tags currently point to from stale ones. Push only
latest-{arch} and record the Dockerfile hash as an image label; the
skip-rebuild check reads the label back via buildx imagetools.

.gitea/workflows/ci.yml

@@ -24,18 +24,14 @@ jobs:
       - name: Build and push image if changed
         run: |
           image=git.weaselab.dev/weaselab/conflict-set-ci
-          tag="$(sha256sum Dockerfile .pre-commit-config.yaml | sha256sum | cut -c 1-16)-${{ matrix.arch }}"
-          latest=latest-${{ matrix.arch }}
-          if docker manifest inspect "$image:$tag" > /dev/null 2>&1; then
-            if [ "$(docker manifest inspect "$image:$tag")" = "$(docker manifest inspect "$image:$latest" 2> /dev/null)" ]; then
-              echo "$image:$latest is up to date"
+          hash="$(sha256sum Dockerfile .pre-commit-config.yaml | sha256sum | cut -c 1-16)"
+          latest="$image:latest-${{ matrix.arch }}"
+          current="$(docker buildx imagetools inspect "$latest" \
+            --format '{{index .Image.Config.Labels "dev.weaselab.ci-hash"}}' 2> /dev/null || true)"
+          if [ "$current" = "$hash" ]; then
+            echo "$latest is up to date"
           else
-              docker pull "$image:$tag"
-              docker tag "$image:$tag" "$image:$latest"
-              docker push "$image:$latest"
-            fi
-          else
-            docker build --push -t "$image:$tag" -t "$image:$latest" .
+            docker build --push --label "dev.weaselab.ci-hash=$hash" -t "$latest" .
           fi
 
   pre-commit: