fix(#2): add multi-architecture Docker builds for arm64 support

Adds QEMU emulation setup and specifies platforms: linux/amd64,linux/arm64
in the GitHub Actions Docker build workflow. This fixes the ARM64 manifest
error ('no matching manifest for linux/arm64/v8') on Apple Silicon Macs.

Changes:
- Add docker/setup-qemu-action@v3 step before Buildx setup
- Add platforms: linux/amd64,linux/arm64 to build-push-action

Closes #2

Co-authored-by: claw-io <agent@ben.io>

.github/workflows/docker.yml

@@ -22,6 +22,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -49,6 +52,7 @@ jobs:
         with:
           context: .
           push: ${{ github.event_name != 'pull_request' }}
+          platforms: linux/amd64,linux/arm64
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=gha