Rehome libcxx-builder docker image & attempt gentler termination. (#71604)

There are three changes present in this PR.

1. Use github packages for libcxx-builder rather than dockerhub.

The ldionne/libcxx-builder image will now be hosted at
ghcr.io/libcxx/libcxx-builder. This has the benefit of allowing members
of the github org to push new versions.

In the future I hope to add github actions to rebuild the image as
needed.

2. Add docker-compose file

The compose file allows to to specify the package repository, so that
users can simply write 'docker compose build' and 'docker compose push'.

It also gives us a centralized place to manage version arguments which
change frequently.

3. Use non-shell CMD form.

This may help the google libcxx builders disconnect more gracefully as
the shell form of CMD may eat the shutdown signal. I'm hoping this
corrects inaccurate agent counts from the buildkite API, since when the
VM's terminate, they do so without signaling it to buildkite, which
hangs around waiting for them to reconnect. It's likely more changes
will be needed though.

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Co-authored-by: Mark de Wever <zar-rpg@xs4all.nl>