Add option to auto upgrade user tables

Hi,
from todays point, it is possible to automatically run mariadb-upgrade --upgrade-system-tables on container start, by setting MARIADB_AUTO_UPGRADE to TRUE.

Unfortunately, this is not covering my user tables, which leads to the fact, that theres still a human interaction required, to complete the MariaDB upgrade.

I am sure, that this is not the expected behavior for important production deployments, but for smaller projects/home use, it would be very nice, to have an optional way/variable, to also upgrade the user tables on container startup.

From my recent lookup in the current docker-entrypoint.sh, this can may be handled with only one additional if condition - but i am not sure, whether there is any greater impact or something different, that must be kept in mind, while implementing this feature.

If the maintainers are fine with a feature like that, i may also contribute it via PR.

Start in docker-entrypoint.sh and trace the existing MARIADB_AUTO_UPGRADE handling and its mariadb-upgrade --upgrade-system-tables invocation. Check how container startup options are documented and tested, then define the optional behavior for upgrading user tables without changing the existing system-table upgrade path. Done means the opt-in setting completes both upgrades during startup.