Update image to work with the current mariadb docker images

[PReimers]

The current MariaDB image uses the mariadbd command instead of the "old" mysqld command.

In addition, I added a random delay (6-60 seconds) in the galera.sh script. This is needed when creating a new docker service, else the peers would be trapped in an endless restart loop because there are not enough peers ready to create a cluster.