feat: add linux/arm64 support to published Docker image

Multi-arch build via QEMU emulation on ubuntu-latest. Publishes a
manifest list with both linux/amd64 and linux/arm64 so users on
Apple Silicon and Graviton can pull and run natively.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: kderholtvisma

.github/workflows/docker-publish.yml

@@ -61,10 +61,17 @@ jobs:
             org.opencontainers.image.description=MCP server for GitHub Actions - lookup actions and get SHA-pinned references
             org.opencontainers.image.licenses=MIT
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@ce360397dd3f832beb865e1373c09c0e9f86d70a # v4.0.0
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@4d04d5d9486b7bd6fa91e7baf45bbb4f8b9deedd # v4.0.0
+
       - name: Build and push Docker image
         uses: docker/build-push-action@263435318d21b8e681c14492fe198d362a7d2c83 # v6.18.0
         with:
           context: .
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}