cannot connect to the docker daemon is the docker daemon running on this host?

ubuntu@s2-umem:~$ sudo service docker status
● docker.service
   Loaded: loaded (/etc/systemd/system/docker.service; enabled; vendor preset: enabled)
   Active: failed (Result: start-limit) since Thu 2017-04-06 20:19:41 UTC; 3min 14s ago
  Process: 27480 ExecStart=/usr/bin/dockerd -H tcp://0.0.0.0:2376 -H unix:///var/run/docker.sock --storage-driver aufs --tlsverify --tlscacert /etc/docker/ca.pem --tlscert /etc/docker/server.pem --tlskey /etc/docker/server-key.pem --label provider=amazonec2 (code=exited, status=203/EXEC)
 Main PID: 27480 (code=exited, status=203/EXEC)

Apr 06 20:19:41 s2-umem systemd[1]: Started docker.service.
Apr 06 20:19:41 s2-umem systemd[27480]: docker.service: Failed at step EXEC spawning /usr/bin/dockerd: No such file or directory
Apr 06 20:19:41 s2-umem systemd[1]: docker.service: Main process exited, code=exited, status=203/EXEC
Apr 06 20:19:41 s2-umem systemd[1]: docker.service: Unit entered failed state.
Apr 06 20:19:41 s2-umem systemd[1]: docker.service: Failed with result 'exit-code'.
Apr 06 20:19:41 s2-umem systemd[1]: docker.service: Start request repeated too quickly.
Apr 06 20:19:41 s2-umem systemd[1]: Failed to start docker.service.
Apr 06 20:19:41 s2-umem systemd[1]: docker.service: Failed with result 'start-limit'.

Solution, open the file /etc/systemd/system/docker.service and replace the line:

ExecStart=/usr/bin/dockerd -H tcp://0.0.0.0:2376 -H unix:///var/run/docker.sock --storage-driver aufs --tlsverify --tlscacert /etc/docker/ca.pem --tlscert /etc/docker/server.pem --tlskey /etc/docker/server-key.pem --label provider=amazonec2

With:

ExecStart=/usr/bin/docker daemon -H tcp://0.0.0.0:2376 -H unix:///var/run/docker.sock --storage-driver aufs --tlsverify --tlscacert /etc/docker/ca.pem --tlscert /etc/docker/server.pem --tlskey /etc/docker/server-key.pem --label provider=amazonec2