Add the possibility of specifying a subnet for --insecure-registry

Signed-off-by: Tibor Vass <teabee89@gmail.com>

Fixes #9038

Ping @jbeda @proppy @dmcgowan @unclejack @fredlf @SvenDowideit @jamtur01 
Need a comma after "Therefore". I also think it would be clearer if we make this active voice. Something like: "Therefore, running `docker -d --insecure-registries 10.20.0.0/16` would allow you to use `docker push` and `docker search` commands with any registry whose IP is part of the 10.20.0.0/16 subnet without issues."

Lastly, we should explain a little what we mean by "work properly".