[hypriot] docker encounter "failed to start docker service"

Today I tried to connect to my hypriot cluster with node-2, I found it always failed to connect.

After did some investigation, I found there is no docker daemon.   I tried to restart docker daemon

it only  claim Job for docker.service failed and suggest to execute jounalctl -xn for details

Finally, I just found it just a resident docker.pid existed in /var/run.  After I delete it and restart the 

system, everything goes well.

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sudo service docker restart
Job for docker.service failed. See 'systemctl status docker.service' and 'journalctl -xn' for details.
;//runHypriotOS: root@cl-node-2 in /var/run