fix: build multi-arch images for amd64 and arm64

Add QEMU setup and build for linux/amd64 and linux/arm64 (armv8), publishing
a single multi-arch manifest. The nginx-unprivileged base image provides both
architectures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -93,6 +93,9 @@ jobs:
           echo "release=${release}" >> "$GITHUB_OUTPUT"
           echo "Computed bump=${bump}, release=${release}, base=${base}"
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -108,6 +111,7 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: .
+          platforms: linux/amd64,linux/arm64
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.version.outputs.tags }}
           labels: |