Generate and distribute official Docker images

Rationale:

• ATM our "official" Docker distributions are generated by a contrib project at https://github.com/xwiki-contrib/docker-xwiki (and the release process documents it at http://dev.xwiki.org/xwiki/bin/view/ReleasePlans/ReleasePlanHelp#HUpdateDocker
• Since this is about the XWiki project's official docker images, it would be better if they could be generated inside the platform distribution modules and generated using a Maven build.

See http://markmail.org/message/4ncp4f4sw3fkd2bn