Write on-the-fly docker image for Postgres

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

Target versions are 9.3, 9.4, 9.5, 9.6, one image per version.

Database can be initialized through SQL, see here:
https://stackoverflow.com/questions/33309121/using-docker-compose-to-create-tables-in-postgresql-database

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

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

Target folder:
docker/images/postgresql

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

Add compose file for simple Nuxeo + DB scenario.