Automate publishing Docker Images to DockerHub/GitHub Container Registry

[ClariNerd617]

Feature request

#2229 and #895 both ask for Docker images in Docker Hub.

Summary

At this point in time, the official docker image for DevDocs is well over five years old, and thus pulling it makes me feel nervous. I would like to be able to self-host DevDocs, so it would make sense to add a GitHub Action to automatically build and push images from time to time.

If necessary, I can fork this, figure this out, and open a PR, along with updated docs (such as for using docker/podman and/or compose.