feat: add multi-arch Docker builds for Raspberry Pi support

Add QEMU and Buildx to CI workflow to build images for linux/amd64,
linux/arm64, and linux/arm/v7 platforms. Closes #29 discussion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/docker-publish.yml

@@ -56,6 +56,12 @@ jobs:
           echo "Generated tags: $TAGS"
           echo "tags=$TAGS" >> $GITHUB_OUTPUT
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
       - name: Log in to Docker Hub
         uses: docker/login-action@v3
         with:
@@ -75,4 +81,5 @@ jobs:
           context: .
           file: ./Dockerfile
           push: true
+          platforms: linux/amd64,linux/arm64,linux/arm/v7
           tags: ${{ steps.tags.outputs.tags }}