On: CoreOS is building a container runtime, Rocket

[cpuguy83]

Docker can import any tarball as a rootfs for a container, essentially allowing you to use whatever build tool you want.

Dockerfiles/`docker build` is an implementation of a build system which uses the docker engine to make said rootfs.

[ash → cpuguy83]

Yes, but the actual container image that is being distributed can only be created by Docker. The ability to import is nice, but irrelevant here.