docker Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

Warning: failed to get default registry endpoint from daemon (Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?). Using system default: https://index.docker.io/v1/

Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?