They are currently not available to end users. This would be a cool feature to have though (and it has been asked for a few times already).
There is a concrete proposal for how to implement it in
A Pull Request to the https://github.com/jupyter/repo2docker repository that implements MIn’s idea from his comment would be great and (I think) suitable as a first contribution. You probably need to know (or want to learn about) how Docker and Docker entrypoints work. Do you want to give it a go?