This is related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3831

It:

- adds some missing variables
- version variable to branch variable conversion

While it improves the situation, it still doesn't result in a working
self-building feature, because:

- cloning a repository containing submodules requires special care,
  as described here
  (c03ef20e5d/README.md (initial-build-setup))

- we likely better clone a specific repository version from the very beginning, instead of trying to change subsequently (and having to re-initilize submodules)
  though that's very different than what we're doing elsewhere and is likely very wasteful

- we can't just build a `Dockerfile`, we need to invoke `make setup` (`./setup.sh`)

- it's likely that the `docker-src` directory is not reusable across
  versions or may require special handling to clean up files, etc.