moby/libnetwork/docs/vagrant-systemd/docker.service

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

[Service]
EnvironmentFile=-/etc/default/docker
ExecStart=/usr/bin/docker daemon -H fd:// $DOCKER_OPTS
MountFlags=slave
LimitNOFILE=1048576
LimitNPROC=1048576
LimitCORE=infinity

[Install]
WantedBy=multi-user.target