> I'm aware of runC but don't know if Docker images are realistically portabl

(It wasn't clear from your comment if you were aware of this) Since last year (docker 1.11) docker itself no longer is a runtime, and uses runC as the default runtime (https://blog.docker.com/2016/04/docker-engine-1-11-runc/)

Additional OCI compliant runtimes can be configured on the daemon (https://docs.docker.com/engine/reference/commandline/dockerd...), and can be selected per container, using the "--runtime" option on "docker run" (https://docs.docker.com/engine/reference/commandline/run/#op...)