Expose conf file for mariadb

[ajstanley]

Drupal's preference is to have the isolation level for transactions set to READ-COMMITTED instead of the default REPEATABLE-READ. If the config file were exposed and persistable we could change that default, and allow some performance tuning

[nigelgbanks]

Environment variables can be added like the existing ones.

https://github.com/Islandora-Devops/isle-buildkit/tree/main/mariadb#settings
https://github.com/Islandora-Devops/isle-buildkit/blob/main/mariadb/rootfs/etc/confd/templates/mariadb-server.cnf.tmpl#L11

Alternatively, if you want to just provide your own file, you can mount it at /etc/confd/templates/mariadb-server.cnf.tmpl and that file will be used for /etc/my.cnf.d/mariadb-server.cnf when the template renders.