moby/contrib/init/systemd/docker.socket

[Unit]
Description=Docker Socket for the API

[Socket]
# If /var/run is not implemented as a symlink to /run, you may need to
# specify ListenStream=/var/run/docker.sock instead.
ListenStream=/run/docker.sock
SocketMode=0660
SocketUser=root
SocketGroup=docker

[Install]
WantedBy=sockets.target