ci(publish): build & push multi-arch images (amd64 + arm64) to GHCR

Add docker/setup-qemu-action and platforms: linux/amd64,linux/arm64 to
both the backend and nginx build-push steps so GHCR images run natively
on arm64 (ARMv8) hosts instead of amd64-only. Refs #56.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/publish.yml

@@ -42,6 +42,9 @@ jobs:
             type=semver,pattern={{major}}.{{minor}}
             type=sha,prefix=sha-,format=short
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
 
@@ -50,6 +53,7 @@ jobs:
         with:
           context: ./backend
           file: backend/Dockerfile
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
@@ -85,6 +89,9 @@ jobs:
             type=semver,pattern={{major}}.{{minor}}
             type=sha,prefix=sha-,format=short
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
 
@@ -93,6 +100,7 @@ jobs:
         with:
           context: .
           file: nginx/Dockerfile
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}