Build multi-arch images so pulls work regardless of runner/host CPU

The first successful Gitea-side publish only produced a manifest for
whatever architecture the self-hosted Actions runner happens to be,
which didn't match the amd64 ZimaOS host pulling it ("no matching
manifest for linux/amd64"). Adds QEMU + Buildx setup and builds for
linux/amd64 and linux/arm64 explicitly so the published image works on
any host regardless of what CPU built it.

.github/workflows/publish-container.yml

@@ -29,6 +29,12 @@ jobs:
             echo "image=$host/marvin/vboxstock" >> "$GITHUB_OUTPUT"
           fi
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Buildx
+        uses: docker/setup-buildx-action@v3
+
       - name: Sign in to container registry
         uses: docker/login-action@v3
         with:
@@ -51,6 +57,7 @@ jobs:
         with:
           context: .
           push: true
+          platforms: linux/amd64,linux/arm64
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           build-args: |