The open-source Docker application virtualization container project has become a defacto standard for applications containers over the course of the last year. But it’s a defacto standard that isn’t a real specification and is one that is being challenged by Linux distribution vendor CoreOS.
CoreOS is developing its own Docker replacement, 
both in terms of technology with Rocket and in terms of an actual specification for defining what an application container is all about. The App Container Specification (appc) is currently in development and has some important differences in contrast to Docker.