feat: Enable multi-architecture Docker image builds (amd64, arm64)

Updates the GitHub Actions workflow (`.github/workflows/docker-publish.yml`)
to build and push Docker images for both `linux/amd64` and `linux/arm64`
architectures.

The `platforms` attribute has been added to the `docker/build-push-action`
step for both the backend and frontend jobs. This ensures that you on
different CPU architectures can use the published images from ghcr.io.