Jupyterlab workspace: implement gosu-helper functionality or equivalent

[jonatanschroeder]

Recently, while trying out the jupyterlab-python workspace locally, I had some issues with file permissions. Namely, I was unable to save the workbook, since the file was created/mounted as root by the workspace host, but accessed as a local user (jovyan) in the Jupyterlab environment. This is handled in other workspaces (if I understand it correctly) with a script by @echuber2 that properly handles the permissions locally, but this is not implemented in this workspace.

Related: #6998.