Automatically mount a empty dir under /home/jovyan when testing

[yuvipanda]

When running on a JupyterHub, a persistent directory is mounted on $HOME (/home/jovyan in this case). This means any files put under $HOME during the image build process will not be available during image run. $HOME should be empty at the end of the image build.

The testing instructions in the README should be updated to mount an empty / temporary volume on /home/jovyan to replicate this, so there's full 1:1 behavioral mapping between how the image will be run on the hub and testing locally.