Update the `jupyterhub/repo2docker` image

[jtpio]

Update the jupyterhub/repo2docker Docker image to point to a pinned version by default.

The tag is made configurable in case so it's easier to override if needed, without having to update this repo.

Having a default pinned version makes it easier to reason about which version is being installed when the playbook is run multiple times.

Fixes https://github.com/plasmabio/plasma/issues/211

• [x] Pick the latest stable tag as default: 2023.06.0-41.g57d229e
• [x] Add / update the documentation: https://plasmabio--217.org.readthedocs.build/en/217/
• [x] Fix custom entrypoint with the newer repo2docker
• [x] Fix tests

[jtpio]

Environments seems to be building fine locally:

Although servers time out on startup. Wondering if it could be related to the custom entrypoint used in Plasma, and the fact that it has since changed in repo2docker to be implemented in Python instead of being a Bash script: https://github.com/jupyterhub/repo2docker/pull/1014

tornado.web.HTTPError: HTTP 500: Internal Server Error (Spawner failed to start [status=0]. The logs for jtp may contain details.)

[atrawog]

This issue is quite likely caused by https://github.com/jupyterhub/repo2docker/pull/1287 and the latest version of repo2docker using Ubuntu 22.04 instead of 18.04 and should be fixed by using repo2docker --build-arg BASE_IMAGE=ubuntu:18.04 or adding BASE_IMAGE=ubuntu:18.04 to the build args in the tljh-repo2docker web gui when craeting the environment.