I don't think that you can compare CoreOS with Docker. For my understanding Docker is a container runtime and CoreOS is an operating system that provides all components you need to run a container runtime. You can compare docker with rocket!
I never used rkt and I used docker just for testing. Rkt is backed by Google Vmware, Red Hat ... and they say that rkt provides better standard container than docker. cloudtweaks.com/2015/03/docker-vs-rocket-containe…