Rocket actually isn't a fork of Docker. From https://coreos.com/blog/rocket/ :

  > Why not just fork Docker?

  > From a security and composability perspective, the 
  > Docker process model - where everything runs through a 
  > central daemon - is fundamentally flawed. To “fix” 
  > Docker would essentially mean a rewrite of the project, 
  > while inheriting all the baggage of the existing 
  > implementation.