So I asked where I can find the source code of # Hub on their subreddit, and rather than allowing for unfavorable discussion, they censored it:

https://www.reddit.com/r/docker/comments/5r67ih/location_of_docker_hub_source_code/

Despite Docker being free software, and their registry server being free, Docker Hub (the React program that's a UI atop of the registry) is # While you can use the registry from their CLI tool, you cannot register an account without running non-free #

They apparently have a GitHub "project" for submitting issues; that project has no source code. I added an issue:

https://github.com/docker/hub-feedback/issues/946

I don't use Docker personally.