[bug] Remove `jovyan` user from Jupyter Dockerfile

Describe the bug

QHub uses actual usernames instead of the conventional jovyan user, but that user still exists in the base Jupyterlab Dockerfile and has its own home directory in addition to the actual username: https://github.com/Quansight/qhub/blob/main/qhub/template/image/Dockerfile.jupyterlab#L11

This results in some unexpected behavior when running basic commands:

• creation of ssh keys appear exclusively in /home/jovyan folder, irrespective of user
• other commands e.g. ls hit the /home/{username} folder instead

Expected behaviour

Any and all commands should be run inside the user’s /home/{username} folder, and /home/jovyan should not exist.

[magsol]

We discussed this at our stand-up this morning and it sounds like it’s a fairly straightforward fix to remove jovyan entirely. I believe @tylerpotts @iameskild are tasked with it, and @costrouc will review the resulting PR.

[costrouc]

Closes by PR #1089