How does volumes work with dockerized jupyterhub?

Hi, I have a quick question but cannot find answer anywhere.

If I have jupyterhub running inside docker container (like e.g. https://hub.docker.com/r/jupyterhub/jupyterhub/) and it uses DockerSpawner that spawns user server as another docker container (e.g. https://hub.docker.com/r/jupyter/datascience-notebook) and inside my jupyterhub_config.py I add:

c.DockerSpawner.volumes = { '/home/XXX/mynotebooks/':notebook_dir }

I know that notebook_dir is inside container used by user, but where is /home/XXX/mynotebooks/?
Is it inside container with jupyterhub or on host computer?