Write on-the-fly docker image for MongoDB

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

Target versions are 2.8, 3.0, 3.2, 3.4, one image per version.

Database can be initialized through Mongo command line. (see docker repo with examples).
Additional help here:
https://www.tutorialspoint.com/mongodb/index.htm

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

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

Target folder:
docker/images/mongo

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

Add compose file for simple Nuxeo + DB scenario.