Zulip Container tries to connect to postgres as user 'zulip' regardless of if postgres user is different in docker-compose.yml

## Scenario:

In `docker-compose.yml` you set:

DB_USER to something other than 'zulip'

## Expcected Result:

Zulip should try to connect to the postgres database as whatever you have set DB_USER to.

## Actual Oucome

Zulip container exits straight away as it can't connect to database as the user 'zulip' doesn't have permissions.