Add ability to change mariadb root host with environment variable

[jez500]

Request

It would be great if we could change the mariadb root host with an environment variable like you can on the official mariadb images
eg. https://mariadb.com/kb/en/mariadb-docker-environment-variables/#mariadb_root_host-mysql_root_host. The default appears to be root@localhost which is fine and secure, but the option to change it would be useful.

Reasoning

We use parallel tests, this requires root@% to create databases, eg laggon_1, lagoon_2, etc. This cannot be done with root@localhost

Workaround

We have added an additional entrypoint that adds root@% but it is a bunch of extra code that could be avoided.