The application container specification (ACI) and the initial implementation, Rocket, are here to provide balance to the container ecosystem. If you review the draft spec (http://bit.ly/1s4Om8p) you can see it's clear that the proposed specification is designed to remove the need for a "vendor" when it comes to building, distributing, and running containers. As an example everything in the spec can be accomplished using a handful of Linux command line utilities such as gpg, tar, and systemd-nspawn -- you could use lxc or libcontainer as well.
When it comes to distributing container images the ACI spec only requires hosting ACI images (tarballs) behind a HTTP webserver and leverages DNS instead of custom registry APIs. The ACI spec skips the app store model in favor of simplicity -- think RPM and YUM.
docker vs rocket?
Don't really care about that right now. Nothing wrong with Docker providing value, and yes, docker provides tons of value. Rocket is just our (CoreOS) attempt at providing value in a slightly different way. IMO the only people that should "own" the container spec are the people using it, and right now I believe that is the main driver behind our (CoreOS) intentions for proposing the ACI specification.
I know I'm biased by association, but trust me, I want using containers to be a simple thing. In reality using containers should be about as exciting as:
$ yum install app; service start app