Publish image on Docker Hub

[DocDvorak]

Since the docker image for this proxy doesn't include any copyrighted materials, it would be nice to be able to pull this image from Docker Hub for use in Docker Compose deployments.

While a hosted image would provide similar functionality as the current compose.yaml config, the NPX-based approach will take longer to initialize when the container is (re)created, as the NPX cache is not preserved. A cache volume could be added, but this adds additional complexity to the config that could be abstracted away.

If the maintainers of the org and this repo are okay with this proposal, I'm happy to implement the necessary changes to automate image builds/pushes.