Cloud Foundry also quietly forked Docker with Warden/Diego (edit: I meant Garden, thanks kapilvt), although in that case they remained compatible with Docker images.
clearing up some facts.. warden predates docker, its a container impl. diego is something entirely different more like kubernetes or mesosphere (scheduling & health, etc). garden the go implementation of warden containers does add fs compatibility for docker.
Your edit is incomplete. Warden predates Docker and is an independent container system. Diego is a new controller/staging/allocating/health system for part of Cloud Foundry, which is a complete PaaS, of which Warden is a low-level component.
I'm optimistic that the ecosystem as a whole will benefit a lot from this, no matter how much or how little market share Rocket manages to capture.