Populate home dir if it is empty

What docker images this feature is applicable to?

jupyter/base-notebook

What changes do you propose?

Populate home dir if it is empty.

How does this change will affect users?

If a user mounts to /home/{raw_username} it will be populated from /home/jovyan on initial startup.
ℹ️ NB_USER set to {raw_username}; the unescaped username (e.g. from JupyterHub).