ci(docker): build multi-arch (amd64+arm64) image via Build Cloud (#144)

Add a cloud-driver buildx step and platforms: linux/amd64,linux/arm64 so the
published image supports arm64 (jambonz v11 arm64 support). Mirrors the house
pattern (upload-recordings / drachtio-server).

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

.github/workflows/docker-publish.yml

@@ -43,10 +43,18 @@ jobs:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        with:
+          version: "lab:latest"
+          driver: cloud
+          endpoint: "drachtio/freeswitch-builder"
+
       - name: Build and push Docker image
         uses: docker/build-push-action@v4
         with:
           context: .
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.prepare_tag.outputs.image_id }}:${{ steps.prepare_tag.outputs.version }}
           build-args: |