Add Docker working group

/cc @iojs/tc

This is a request to establish a separate Docker working group. This is work that is already being done, and until now has been listed as a responsibility of the Build group. All members of Build who are also currently active contributors to the Docker images (@pesho, @wblankenship) will be among the initial members of the new group.

List of initial members:

@hmalphettes @jlmitch5 @pesho @Starefossen @wblankenship

Also, it would be good to create a new GitHub team (@iojs/docker) with the initial members listed above, and given rights to manage https://github.com/iojs/docker-iojs