don't spawn an extra container
run pg_dumpall within matrix-postgres instead, ensures correct version
store under /matrix so a backup of the folder will contain a DB dump
use absolute paths just in case something in the ENV is messed up

In short, this makes Synapse a 2nd class citizen,
preparing for a future where it's just one-of-many homeserver software
options.

We also no longer have a default Postgres superuser password,
which improves security.

The changelog explains more as to why this was done
and how to proceed from here.

We recently had a report of the Postgres backup container's log file
growing the size of /var/lib/docker until it ran out of disk space.

Trying to prevent similar problems in the future.

We do restart Synapse explicitly, but some other services
(bridges, matrix-corporal, ..) may not restart sometimes.

It's best to restart all services explicitly.