[root@linuxbox ~]# sudo -u postgres createuser confluenceuser -d -P
[root@linuxbox ~]# sudo -u postgres createdb --encoding=utf8 --owner=confluenceuser confluence

[root@linuxbox ~]# su - postgres
Last login: Fri Jun 29 12:29:42 CEST 2018 on pts/0
-bash-4.2$ psql -U postgres
psql (10.4)
Type "help" for help.

postgres=# password confluenceuser
Enter new password:
Enter it again:
postgres=# grant all privileges on database confluence to confluenceuser with grant option;
GRANT
postgres=# alter role confluenceuser with createdb login;
ALTER ROLE
postgres=# alter user confluenceuser valid until 'infinity';
ALTER ROLE
postgres=# list
                                             List of databases
    Name    |     Owner      | Encoding |   Collate   |    Ctype    |          Access privileges
------------+----------------+----------+-------------+-------------+--------------------------------------
 confluence | confluenceuser | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =Tc/confluenceuser                  +
            |                |          |             |             | confluenceuser=C*T*c*/confluenceuser
 postgres   | postgres       | UTF8     | en_US.UTF-8 | en_US.UTF-8 |
 template0  | postgres       | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres                         +
            |                |          |             |             | postgres=CTc/postgres
 template1  | postgres       | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres                         +
            |                |          |             |             | postgres=CTc/postgres
(4 rows)

-bash-4.2$ psql -U confluenceuser
psql: FATAL:  Peer authentication failed for user "confluenceuser"