During development it is sometimes useful to allow remote access to a Docker deamon. When the daemon runs on Ubuntu you'll have to modify /lib/systemd/system/docker.service.
The key thing to do is to add a command line parameter: -H tcp://127.0.0.1:4243 in the Service section of the configuration file.
[Service]
Type=notify
ExecStart=/usr/bin/docker daemon -H tcp://127.0.0.1:4243 -H fd://
Reload the daemon configuration and you are good to go:
$ systemctl daemon-reload
$ /etc/init.d/docker restart