Test both PG (PostgreSQL, Postgres) and MySQL for CE on GitLab CI

Discussion:

Jeroen van Baarsen [10:19 AM] I was wondering, what is the reason we are testing GitLab CE only for MySQL on GitLab CI? Since we say PG is the prefered way of installation

Dmitriy Zaporozhets [10:21 AM] I think its just historically. we tested PG on Semaphore so used CI for mysql

Jeroen van Baarsen [10:21 AM] We can test both now with Docker right?

Kamil Trzciński [10:22 AM] @jvanbaarsen: yes we can