They need to implement support for the Dockerfile format if they want to win. People value inertia. The switching costs have to be low if you expect anyone to switch; this only become non-true when the incumbent becomes intolerably useless to the general userbase.
I agree that we need a better ecosystem of build tools and that is something we are looking to help build out. But, with rkt what we are trying to do is build an excellent runtime; and think the build side is an important and orthogonal problem.
