Allow MariaDB to be used with the local Docker environment

[desrosj]

MariaDB Docker images are ​available in the Docker Hub, and seems that they can be easily substituted for the MySQL ones that the local Docker environment currently uses.

This would unblock #30462, which aims to test multiple database types/versions (though Travis made this really difficult.

[desrosj]

51744.diff​ is a proof of concept that introduces LOCAL_DB_TYPE and accepts mysql or mariadb.

LOCAL_MYSQL is now a bit too specific, but if it is renamed (maybe to LOCAL_DB_VERSION), there should probably be a check that copies the value to the new one.

[davidbaumwald]

Test with mariadb

[desrosj]

51744.2.diff​ changes LOCAL_MYSQL to LOCAL_DB_VERSION.

[desrosj]

In 49836:

Build/Test Tools: Support the use of MariaDB in the local Docker environment.

In addition to MySQL, WordPress also supports MariaDB for databases. This makes changes to the local Docker environment to allow MariaDB to be used instead, if desired.

The LOCAL_DB_TYPE environment variable will now be used to determine which type of database container to use. This will default to mysql, but will also accept mariadb.

The LOCAL_MYSQL environment variable (which previously held the version of MySQL to use) has been replaced with the new, more generic LOCAL_DB_VERSION environment variable.

This change will make it possible to perform automated testing on a wider array of supported database types and versions.

Props davidbaumwald.
Fixes #51744. See #30462.