- The start.sh script does different operations on users, and
  sometimes one may need to do operations *after* that is done (as
  opposed to /usr/local/bin/start-notebook.d/ which is run before).
  This provides that possibility.  (One use case is hooks which must
  be run after the users are set up.)
- To be technically perfect, one might want to use something similar
  to /usr/local/bin/pre-start-notebook.d/.  But let's start with the
  simplest thing possible for now - the expected use of this command
  is only if people use NB_USER, etc in a way that requires small
  follow up commands to be run.
- If notebook is running as root, this allows users to run arbitrary
  commands pre-sudo.  Consider security model.