can't enter through a jupyter app as a first go if container not yet existing

[parmentelat]

as of 0.17.4:

trying to reach e.g.
https://nbhosting-dev.inria.fr/auditor/notebook/mines-python-primer@classic

as the very first URL for a (student, course) couple, i.e. if the container does not yet exist, fails with this message

failed to spawn notebook container
command nbh -d /nbhosting/current -x docker-view-student-course-jupyterdir somebody mines-python-primer
returned with retcod=0 action=failed-unknown-container
<snip>
++ echo 08:16:26 'INTERNAL error: cannot start non-existent container mines-python-primer-x-somebody'

same with @jlab

a student must first enter through an actual notebook, and only then the jupyter apps become available