[bitnami/mediawiki] Add env var to control skipping config validation during startup

[iain-ilearner]

As suggested/requested in https://github.com/bitnami/containers/issues/65721#issuecomment-2078888820 here's a PR introducing a new environment variable that controls whether or not validation is performed of configuration file during database upgrade at container start-up.

This allows the use deprecated values (such as $wgAllowImageTag) in LocalSettings.php

This PR also adds a log message after connecting to the database to better indicate which part of the start-up process is running.