Enable live debuging of JupyterHub container

[vpavlin]

Is your feature request related to a problem? Please describe.
When we develop JupyterHub Singleuser Profiles, we need to rebuild image to test each change, it would be useful to have a way how to update the library and restart JupyterHub without builds and rollouts

Describe the solution you'd like
Provide a script (e.g. devel_update_and_restart.sh) which will update a given dependency and restart JupyterHub without terminating the container (we'd probably need an env var to switch to this mode as well to not disrupt default behaviour)

The script should accept parameters describing the dependency and the version to update to (both PyPi and Git as a source should be supported).

The script would then use pipenv to update that depepndency and restat JupyterHub process

Describe alternatives you've considered
Potential alternative might be to build another layer on top of the JH image, but thet would be very hard to generalize and still would include the image build and container rollout which would take longer than the proposed solution

Additional context