recommend a better way of backing up postgres

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

docs/maintenance-postgres.md

@@ -51,15 +51,12 @@ ansible-playbook -i inventory/hosts setup.yml --tags=run-postgres-vacuum,start
 To make a back up of the current PostgreSQL database, make sure it's running and then execute a command like this on the server:
 
 ```bash
-docker run \
+/usr/bin/docker exec \
---rm \
---log-driver=none \
---network=matrix \
 --env-file=/matrix/postgres/env-postgres-psql \
-docker.io/postgres:13.1-alpine \
+matrix-postgres \
-pg_dumpall -h matrix-postgres \
+/usr/local/bin/pg_dumpall -h matrix-postgres \
 | gzip -c \
-> /postgres.sql.gz
+> /matrix/postgres.sql.gz
 ```
 
 If you are using an [external Postgres server](configuring-playbook-external-postgres.md), the above command will not work, because the credentials file (`/matrix/postgres/env-postgres-psql`) is not available.