JupyterHub apiUrl detection fails when hub.baseUrl is set

[TomAugspurger]

The Jupyterhub helm chart includes a hub.baseUrl for serving the hub under a path other than /, e.g. /jupyter/. I believe this breaks the automatic API setting at https://github.com/dask/dask-gateway/blob/7d1659db8b2122d1a861ea820a459fd045fc3f02/resources/helm/dask-gateway/templates/gateway/configmap.yaml#L97.

I'm not sure if the baseUrl is available where that code is running, so it may be hard to detect the url automatically.

The workaround is to explicitly set the URL

gateway:
  auth:
    jupyterhub:
      apiUrl: http://proxy-http:8000/jupyter/hub/api

[consideRatio]

Hmmmmm, I think the right call is to document the need to explicitly configure gateway.auth.jupyterhub.apiUrl if JupyterHub has a custom hub.baseUrl, fullnameOverride, or nameOverride.