Many thanks for this helpful hint on resetting the root password. However, on my MariaDB version (11.2.2) adding the post argument "mysqld_safe --skip-grant-tables" resulted in the container no longer wanting to start. The following line was then found in the log file:
/usr/local/bin/docker-entrypoint.sh: line 658: exec: mysqld_safe: not found
I then discovered in the MariaDB documentation that something had changed in the symlinks for this command as of version 10.5.2. I therefore used the command "mariadbd-safe" instead of "mysqld_safe" and then everything worked perfectly. I'm not sure if this is the right solution, but at least it did the trick.
