Improve Dockerfile

This isn't as performant as it could be. The multi-stage build could be restated.

And practice good hygiene.

- https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#entrypoint
- https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#user
- https://snyk.io/blog/how-and-when-to-use-docker-labels-oci-container-annotations/