Since upgrading to docker-17.0.3.1 I cannot start containers: $ docker run --rm hello-world container_linux.go:247: starting container process caused "process_linux.go:334: running prestart hook 0 caused \"fork/exec /var/tmp/portage/app-emulation/docker-17.03.1/image/usr/bin/dockerd (deleted): no such file or directory\"" docker: Error response from daemon: oci runtime error: container_linux.go:247: starting container process caused "process_linux.go:334: running prestart hook 0 caused \"fork/exec /var/tmp/portage/app-emulation/docker-17.03.1/image/usr/bin/dockerd (deleted): no such file or directory\"". It appears to be trying to run dockerd out of the /var/tmp/portage.
I'm closing this out. After upgrading I hadn't restarted the docker daemon. After re-starting it now appears to work.