Openshift 3.0 is built on top of Kubernetes. Docker images is actually a small part of OpenShift and will be likely replaced by OCI in the future whenever it will be production ready. RedHat is a main contributor of Kubernetes. Kubernetes currently support Docker and RKT images. It is important to mention Docker since Openshift should run Docker images, however it is also very important to mention Kuberneres since Openshift should support also any Kubernetes based containers orchestration.