This allows people to try out the new Element X clients, which need to
run against the sliding-sync proxy (https://github.com/matrix-org/sliding-sync).
Supersedes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2515
The code is based on the existing PR (#2515), but heavily reworked. Major changes:
- lots of internal refactoring and variable renaming
- fixed self-building to support non-amd64 architectures
- changed to talk to the homeserver locally, over the container network (not
publicly)
- no more matrix-nginx-proxy support due to complexity (see below)
- no more `matrix_server_fqn_sliding_sync_proxy` in favor of
`matrix_sliding_sync_hostname` and `matrix_sliding_sync_path_prefix`
- runs on `matrix.DOMAIN/sliding-sync` by default, so it can tried
easily without having to create new DNS records
