CG-3 is dropping support for the Ubuntu 12.04 and CentOS/RHEL 6 distros. They are getting to be too old and need special cases to handle their quirks.
Ubuntu has released two LTS versions since 12.04, so everyone can easily upgrade or use Docker. Similarly, EL7 has been out for over two years. The older distros are also scheduled to go EOL in 6 months, so if you haven't upgraded already, you must do so shortly anyway.
-- Tino Didriksen