Try to avoid Docker logs growing too much for one-off containers

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.

docs/maintenance-postgres.md

@@ -42,6 +42,7 @@ To make a back up of the current PostgreSQL database, make sure it's running and
 ```bash
 docker run \
 --rm \
+--log-driver=none \
 --network=matrix \
 --env-file=/matrix/postgres/env-postgres-psql \
 postgres:12.4-alpine \