Completely untie the backend from the frontend.

Build nginx and mariadb together with the app image.
Upgrade docker-compose configuration to 3 version.
Add cleanup step to the gitlab-ci.

docker/phpmyadmin/config.inc.php

@@ -0,0 +1,4 @@
+<?php
+
+$cfg['NavigationTreeEnableGrouping'] = false;
+$cfg['LoginCookieValidity'] = 60 * 60 * 24 * 365;