Add POSTGRES_PASSWORD and USER, with warning on no password

Let's add `-` to this heredoc so we can indent it nicely, ie:
```bash
    gosu postgres postgres --single -E <<-EOSQL
        $TYPE USER '$POSTGRES_USER' WITH SUPERUSER PASSWORD '$POSTGRES_PASSWORD'
    EOSQL
    { echo; echo "host all '$POSTGRES_USER' 0.0.0.0/0 md5"; } >> "$PGDATA"/pg_hba.conf
```