Add no-new-privileg flag

The daemon config for defaulting to no-new-privileges for containers was
added in d7fda019bb7e24f42f8ae1ddecb3fd52df3c48bf, but somehow we
managed to omit the flag itself, but also documented the flag.
This just adds the actual flag.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

docs/reference/commandline/dockerd.md

@@ -71,6 +71,7 @@ Options:
       --max-concurrent-uploads int            Set the max concurrent uploads for each push (default 5)
       --metrics-addr string                   Set default address and port to serve the metrics api on
       --mtu int                               Set the containers network MTU
+      --no-new-privileges                     Set no-new-privileges by default for new containers
       --oom-score-adjust int                  Set the oom_score_adj for the daemon (default -500)
   -p, --pidfile string                        Path to use for daemon PID file (default "/var/run/docker.pid")
       --raw-logs                              Full timestamps without ANSI coloring