create dockerhub action

[jasmainak]

This is more as a reminder for me. We can create an action that converts a Dockerfile into a docker image and uploads it to dockerhub:

https://github.com/jupyterhub/repo2docker-action

The advantage would be two fold:

• it can be reused in both the binder links and in HNN workshops. The time
• the docker installation will be (partially) continuously tested and the action will fail if something in the install fails ... we don't end up with the situation where we have to spend 2 days to create a working dockerfile for a workshop as we're fixing things on the go