Ghost officially supports MySQL, and since Ghost V5, this has become a clear requirement–ghost update issues a notice if MariaDB is found. According to other MariaDB users, the latest update has a breaking change.
For some time, MariaDB and MySQL have diverged. Moreover, Ghost never officially support MariaDB. Your best workaround is to downgrade to Ghost 5.20, which appears to work with MariaDB. Long-term, you need to install MySQL 8.