/ docker

Enable Remote API on Docker hosts running systemd (like Ubuntu 15.04)

In linux distros running systemd, like Ubuntu 15.04, adding -H tcp:// to /etc/default/docker does not have the effect it used to.

Instead, create a file called /etc/systemd/system/docker-tcp.socket to make docker available on a TCP socket on port 2375:

Description=Docker Socket for the API

Then enable this new socket:
systemctl enable docker-tcp.socket

systemctl enable docker.socket

systemctl stop docker

systemctl start docker-tcp.socket

systemctl start docker

Test that it's working:
docker -H tcp:// ps

Source: https://coreos.com/docs/launching-containers/building/customizing-docker/