Add devcontainer and Github Codespaces setup

[cleverer]

Just works ;) (If you don't know how, check the docs)

After merging it would make sense to set up prebuilds: https://docs.github.com/en/codespaces/prebuilding-your-codespaces/configuring-prebuilds

I'd recommend to run weekly rebuilds I guess.

Prebuilds run on Github Actions which is free for public repositories, so no additional costs. Only storage for the prebuilt images is not enough with 15GB…

[cleverer]

This PR has been reworked and optimized. Thanks to @BacLuc, @petter1 and all the others for their inputs and help!

New since the last time:

• Docker containers now wait for each other using docker healthchecks. This makes the waitfortcp in the entrypoints obsolete.
• Github SSH Host keys are preregistered in the docker container, simplifying pulling via ssh.
• Replace mysql with corresponding mariadb. (Yay arm-based machines 🎉 )
• Don't mount the whole home directory outside the container. That leads to a lot of confusing behaviour…
• Don't mount the X11 socket by default, as it causes issues on multiple platforms (arch, macos on Apple Silicon…)
• Allow to access docker from within the dev container

Currently not working:

• If you run the Github Codespace in the Browser and access hitobito, CORS is not set up correctly.
• Debugging inside the container requires a merge of fix(Gemfile): pin json gem below 2.7.0 hitobito#2478
• The prebuilt images currently seem a bit finicky. If in doubt do a full rebuild as described in docs/Devcontainer.md and grab a coffee ;)

TODOs for Puzzle

• After merging this, it would be wonderful if pre-builds would be setup, for that truly instant first time setup.
• It would be lovely if a .github Repository could be created on the hitobito org. There you could add links like this:
  • https://github.com/codespaces/new?ref=devcontainer&repo=277813823&machine=basicLinux32gb&geo=EuropeWest&devcontainer_path=.devcontainer%2Fgeneric%2Fdevcontainer.json
  • This allows people to directly launch a codespace.
  • You can list all possible variants, just change the devcontainer_path param accordingly.
• Update the list of variants/setups aka update .devcontainerconfigs.

If you have questions about how it "just works", check the docs/Devcontainer.md and docs/DevcontainerInDepth.md ;)

@carlobeltrame I think I'm through with reworking, feel free to review and request more docs! Also: Feel free to squash when merging, there's a lot of trial and error here… ^^

[cleverer]

Update for all interested in this: I talked to Puzzle/hitobito, they unfortunately don't have capacity to look at this at the moment. I'll investigate if we can fork this under https://github.com/scout-ch