ci: replace docker/build-push-action with plain docker buildx build

docker/build-push-action's upstream repo has ~260MB of git history, which
reliably hangs the act runner's Go-based git clone on the linux-amd64
(Hostinger) runner — confirmed it's not a network or cache issue: curl to
codeload.github.com and a plain `git clone` of the same repo both complete
in seconds from the host and from inside the exact job container. Only
act's own clone of this specific action hangs, every time.

A raw `docker buildx build` with the same flags does the same job without
cloning anything.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VAyr8d7HuAdUGB6aA5xmDD

.gitea/workflows/release.yml

@@ -75,15 +75,19 @@ jobs:
           registry: ${{ env.REGISTRY }}
           username: ${{ gitea.actor }}
           password: ${{ secrets.REGISTRY_TOKEN }}
+      # Plain `docker buildx build` instead of docker/build-push-action: that
+      # action's upstream repo (~260MB of git history) reliably hangs the
+      # act runner's Go-based clone on the Hostinger host, even when curl/git
+      # to github.com are both fast. Same end result, no action to clone.
       - name: Build and push (per architecture)
-        uses: docker/build-push-action@v7
-        with:
-          context: .
-          platforms: ${{ matrix.platform }}
-          push: true
-          tags: ${{ needs.plan.outputs.image }}:build-${{ gitea.sha }}-${{ matrix.arch }}
-          cache-from: type=registry,ref=${{ needs.plan.outputs.image }}:buildcache-${{ matrix.arch }}
-          cache-to: type=registry,ref=${{ needs.plan.outputs.image }}:buildcache-${{ matrix.arch }},mode=max
+        run: |
+          docker buildx build \
+            --platform "${{ matrix.platform }}" \
+            --push \
+            -t "${{ needs.plan.outputs.image }}:build-${{ gitea.sha }}-${{ matrix.arch }}" \
+            --cache-from "type=registry,ref=${{ needs.plan.outputs.image }}:buildcache-${{ matrix.arch }}" \
+            --cache-to "type=registry,ref=${{ needs.plan.outputs.image }}:buildcache-${{ matrix.arch }},mode=max" \
+            .
       # Runs the image that was just pushed, on its own architecture.
       - name: Smoke test
         run: scripts/smoke-image.sh "${{ needs.plan.outputs.image }}:build-${{ gitea.sha }}-${{ matrix.arch }}"