ci: upgrade docker/build-push-action to v6 and add ARM64 support

Changes:
- Upgrade docker/build-push-action from v5 to v6
- Add multi-platform build support (linux/amd64, linux/arm64)
- Add GitHub Actions cache for faster builds

This allows users on ARM-based machines (Apple Silicon, AWS Graviton,
Raspberry Pi, etc.) to use the official Docker image.

Closes #92
Closes #99

.github/workflows/docker-image.yml

@@ -39,11 +39,14 @@ jobs:
             type=sha
             type=raw,value=latest
 
-      - name: Build and push
-        uses: docker/build-push-action@v5
+      - name: Build and push (multi-platform)
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./Dockerfile
           push: true
+          platforms: linux/amd64,linux/arm64
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max