default home directory vs repo directory

[scottyhq]

cross posting pangeo-data/pangeo-stacks#62:

The build scripts between hubploy (https://github.com/yuvipanda/hubploy/blob/master/hubploy/imagebuilder.py used in pangeo-cloud-federation) and pangeo-stacks (https://github.com/pangeo-data/pangeo-stacks/blob/master/build.py) are slightly different.

An image built with hubploy currently drops a user into a jupyter session in the srv/repo directory https://github.com/yuvipanda/hubploy/blob/master/hubploy/imagebuilder.py#L42

The same image definition built in pangeo-stacks puts a user in /home/jovyan (which is what we want for jupyterhubs and binder).

I'm guessing these directories are best specified at build time. Or is it also okay to specify an environment variable like JUPYTER_SERVER_ROOT in a postBuild script?