> I'm aware of runC but don't know if Docker images are realistically portable
Quay, the Docker Registry competitor from CoreOS, has automatic support for converting Docker images into rkt images (as in: you push the Docker image, and pull with rkt, and it supposedly just works). I don't know how well it works in practice, though I can't imagine (from the top of my head) why it shouldn't. A Docker image is mostly a layered tarball with a few fields of metadata. Nothing particularly obscure.
Quay, the Docker Registry competitor from CoreOS, has automatic support for converting Docker images into rkt images (as in: you push the Docker image, and pull with rkt, and it supposedly just works). I don't know how well it works in practice, though I can't imagine (from the top of my head) why it shouldn't. A Docker image is mostly a layered tarball with a few fields of metadata. Nothing particularly obscure.