Related to:

- https://github.com/matrix-org/rageshake/issues/69
- https://github.com/matrix-org/rageshake/pull/70

BuildX is required for building this container image.

This makes it consistent with the rest of the playbook:
- there's a default config which has various variables controlling
  settings
- there's also an `_extension_yaml` variable, which lets you override it

We disable it via `group_vars/matrix_servers`, as we do for all other
roles.