vendor/github.com/docker/docker/cmd/dockerd/daemon_linux.go

// +build linux

package main

import systemdDaemon "github.com/coreos/go-systemd/daemon"

// notifySystem sends a message to the host when the server is ready to be used
func notifySystem() {
	// Tell the init daemon we are accepting requests
	go systemdDaemon.SdNotify("READY=1")
}