Public docker image live, updated docs

********* PEOPLE *************
Primary reviewer: @suntzu86 

Reviewers:

********* DESCRIPTION **************
Branch Name: sclark_332_set_up_public_docker
Ticket(s)/Issue(s): Closes #332 

********* TESTING DONE *************

Tested on dev and personal linux box
When does OS X need to build manually? Is this supposed to be a 'try the linux way first and then do the slow build if it fails?' I'm still not clear on what's different btwn the two.