Rename env var `TORRUST_TRACKER_DATABASE` to `TORRUST_TRACKER_DATABASE_DRIVER`

The tracker container env var `TORRUST_TRACKER_DATABASE` has been renamed to `TORRUST_TRACKER_DATABASE_DRIVER` on this [commit](https://github.com/torrust/torrust-tracker/commit/7996a9d832f515164ecd075bfd2f6b4712ead2ae). It has to be changed in this repo.