While v1.22.0 supposedly has multi-arch Docker images
(thanks to https://github.com/matrix-org/synapse/pull/7921),
I can't them on Docker Hub yet, so I'm backing out of this change
for now and letting people fall back to self-building there.

Fix docker mount instruction in documentation

Fixes #685 (Github Issue).

The SSL certificates should be accessible to the nginx proxy container, not the synapse container.

also, worker.yaml.j2:
  - hone worker_name
  - remove worker_pid_file entry (would only be used if worker_daemonize
    set to true; also, synapse only knows about the container namespace
    and thus can not provide the required host-view PID)

add variables for secure_backup_required and secure_backup_setup_methods

Discussed in #669 (Github Issue).

Upgrade Element (1.7.8 -> 1.7.9)

Update nginx (1.19.2 -> 1.19.3)

Shameless refactor of all systemd descriptions for bridges

Upgrade Synapse (v1.20.1 -> v1.21.0)

Add docker prune

& restrict federation listener; frontend_proxy / user_dir don't need it