Add multi-arch support (amd64 + arm64) for all Flutter SDK variants

- Add QEMU setup for cross-platform builds
- Build web, android, and linux images for both amd64 and arm64

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitea/workflows/publish.yaml

@@ -45,6 +45,9 @@ jobs:
             echo "tag=${{ matrix.variant }}-latest" >> $GITHUB_OUTPUT
           fi
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -80,7 +83,7 @@ jobs:
         with:
           context: .
           file: ${{ matrix.dockerfile }}
-          platforms: linux/amd64
+          platforms: linux/amd64,linux/arm64
           push: true
           sbom: true
           provenance: mode=max