How to: find the location of systemd unit file
systemctl status docker
#=> docker.service - Docker Application Container Engine
#=> Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)

sudo sed -i.bak 's?ExecStart.*?ExecStart=/usr/bin/dockerd -H tcp://0.0.0.0:2376 -H unix:///var/run/docker.sock?g' /lib/systemd/system/docker.service
sudo systemctl daemon-reload
sudo systemctl restart docker.service