Write on-the-fly docker image for MariaDB

The image would extend the one available here:
https://hub.docker.com/_/mariadb/

Target versions is 10.1, one image.

Database can be initialized through SQL.

More details on Nuxeo specifics here:
https://doc.nuxeo.com/nxdoc/mariadb/

Target repository:
https://github.com/nuxeo/nuxeo-tools-support-priv/tree/master

Target folder:
docker/images/mariadb

The DB version will then be handled in compose files through a MARIADB_VERSION variable (with a default value matching the Nuxeo version).

Add compose file for simple Nuxeo + DB scenario.