Write on-the-fly docker image for MySQL

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

Target versions are 5.6, 5.7, one image per version.

Database can be initialized through SQL.

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

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

Target folder:
docker/images/mysql

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

Add compose file for simple Nuxeo + DB scenario.