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

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Rewrite documentation for insecure registries

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Merge pull request #9100 from tiborvass/insecure-registry-cidr

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