moby/contrib/init/systemd/docker.service

[Unit]
Description=Docker Application Container Engine
Documentation=http://docs.docker.com
After=network.target docker.socket
Requires=docker.socket

[Service]
ExecStart=/usr/bin/docker -d -H fd://
LimitNOFILE=1048576
LimitNPROC=1048576

[Install]
WantedBy=multi-user.target