doc: add Docker WG

A new `Docker` working group is added to `WORKING_GROUPS.md`.
Also, removed the Docker images responsibility from the `Build`
group's description.

PR-URL: https://github.com/iojs/io.js/pull/1134


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>