Add version with MariaDB built-in

[grasdk]

For people not wanting to deal with setting up mariadb or postgres, let's build a single container with both galette and mariadb running. Optionally one can override env-variables to use external Database.

In such a scenario, the mariadb data must be on a mounted volume, so data can be "carried" to the next container when upgrading the galette version

[grasdk]

added a feature branch where this work has been begun. In doubt if I'm on the correct path, or if I should look for a "Docker Apache MySQL PHP" (DAMP) container image instead.

The MariaDB Dockerfile and Entrypoint script are quite advanced: https://github.com/MariaDB/mariadb-docker