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