Error when run Docker on a Container host

I currently working on a project using Containernet (an SDN network using Container as a host) and need to run Docker on it. But I got docker cannot connect to the docker daemon at unix ///var/run/docker.sock when I type docker ps or any Docker commands. Does anybody have an idea or used to run Docker on a Docker Container please tell me? Thank you.
P/S: My Container runs Ubuntu 18.04.