Enabling Docker service to listen on a port

By default the Docker service listens on a local socket. If you want to access the Docker service api remotely, you need to configure the service to listen on a port as well.

On Ubuntu 16.04, edit /lib/systemd/system/docker.service and change this line:

ExecStart=/usr/bin/dockerd -H fd://

to

ExecStart=/usr/bin/dockerd -H fd:// -H tcp://0.0.0.0:2376

Reload the systemd config:

sudo systemctl daemon-reload

and restart the service:

sudo systemctl restart docker.service

More info here.