[QA] occ produces misleading 'Invalid Database' error when run as root

Seen with 10.6.0 on ubuntu 20.04 where the apache user is www-data:

The correct way to call occ, according to e.g. the /usr/bin/occ script shipped in our docker images is

su-exec www-data php /var/www/owncloud/occ ...

If a user runs occ as root, a misleading error message occurs: (The database type is not invalid, the occ invocation is at fault)

cd /var/www/owncloud
./occ app:enable metrics

An unhandled exception has been thrown:
OC\DatabaseException: Invalid database type in /var/www/owncloud/lib/private/Server.php:533
Stack trace:
#0 /var/www/owncloud/lib/composer/pimple/pimple/src/Pimple/Container.php(118): OC\Server->OC\{closure}()
#1 /var/www/owncloud/lib/private/AppFramework/Utility/SimpleContainer.php(108): Pimple\Container->offsetGet()
#2 /var/www/owncloud/lib/private/ServerContainer.php(86): OC\AppFramework\Utility\SimpleContainer->query()
#3 /var/www/owncloud/lib/private/Server.php(1239): OC\ServerContainer->query()
#4 /var/www/owncloud/lib/private/Server.php(412): OC\Server->getDatabaseConnection()

Expected behaviour: occ should print a proper error message, and would refuse to run when called as root user.

(Not sure if this is a regression, didn't we catch that in the past?)

Start with the occ entry point and the database initialization path shown at lib/private/Server.php:533. Reproduce the command as root in /var/www/owncloud, compare it with the www-data invocation from /usr/bin/occ, and trace why the database error is selected. Done means root invocation is refused with a clear error instead of reporting an invalid database type.