fix(ci): add multi-platform Docker build for ARM64 support

Previously Docker image only built for linux/amd64, causing:
  no matching manifest for linux/arm64/v8

Changes:
- Add platforms: linux/amd64,linux/arm64 to build-push-action
- QEMU and Buildx already configured, just needed platforms flag

Fixes #99

.github/workflows/docker-image.yml

@@ -44,6 +44,7 @@ jobs:
         with:
           context: .
           file: ./Dockerfile
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}