I had to edit /lib/systemd/system/docker.service
on my Ubuntu 16.04.2 LTS system to modify the line
ExecStart=/usr/bin/docker daemon -H fd:// -H tcp://0.0.0.0:2375
then
sudo systemctl daemon-reload
sudo systemctl restart docker.service
and everything worked :-). The next step is to figure out how to protect the docker daemon form being hijacked.