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

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

$ service docker start
/etc/init.d/docker: line 35: ulimit: open files: cannot modify limit: Operation not permitted

$ sudo service docker start

$ docker ps -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                      PORTS               NAMES