If you base a Dockerfile on this image:
FROM quay.io/jupyterhub/jupyterhub-onbuild:4.0.2
...
then your jupyterhub_config.py adjacent to your Dockerfile will be loaded into the image and used by JupyterHub.
Note
Inherit from a tag that corresponds to the version of JupyterHub you want to use. See our Quay.io page for the list of available tags.
Warning
Automatically loading the jupyterhub_config.py file was the default behavior of the quay.io/jupyterhub/jupyterhub
image prior to 0.6.