When you’re running Ubuntu 15.04 with Docker, you must use systemctl to enable/disable your Docker service. Unfortunately, the old /etc/default/docker configuration file is not read. You must change the docker.service file and add these lines:
.... [Service] EnvironmentFile=-/etc/default/docker ExecStart=/usr/bin/docker -d -H fd:// $DOCKER_OPTS MountFlags=slave LimitNOFILE=1048576 LimitNPROC=1048576 LimitCORE=infinity ....
(On my Ubuntu 15.04 system, the file resides at /etc/systemd/system/multi-user.target.wants/)
Restart Docker with:
$ sudo systemctl restart docker
Brain/Drain/Pain 