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

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

In linux distros running systemd, like Ubuntu 15.04, adding -H tcp://0.0.0.0:2375 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:

[Unit]
Description=Docker Socket for the API
[Socket]
ListenStream=2375
BindIPv6Only=both
Service=docker.service
[Install]
WantedBy=sockets.target

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://127.0.0.1:2375 ps

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