Cannot connect to the Docker daemon. Is the docker daemon running on this host?

Docker Engine Swarm
17

For Ubuntu 16.04

Inside file /lib/systemd/system/docker.service change:
ExecStart=/usr/bin/dockerd fd://
with
ExecStart=/usr/bin/dockerd -H tcp://0.0.0.0:2375

Inside file /etc/init.d/docker change:
DOCKER_OPTS=
with
DOCKER_OPTS="-H tcp://0.0.0.0:2375"

and then restart your computer.