Not sure why this is necessary, perhaps due to our use of a minimal
init process.

Closes #1

set HOME correctly, and don't set hostname from within the container
(this fails).

Without these 'jupyter' will not be in the path and everything will
explode.

The price to pay is to run `start-singleuser.sh` as NB_USER, which
takes advantage of the fact that `start.sh` does not try to rename
users and home directories if not run as root.

Closes #2

The supervisord.conf file can use environment variables
with the "%(ENV_VARNAME)s" syntax.

See: https://stackoverflow.com/questions/12900402/supervisor-and-environment-variables

The script uses bash constructs ([[) that the bourne shell
does not know about