Fixed #6545 - Updated Security article

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
I'm not sure it it's correct to say that we drop all by default.  Maybe a link the the whitelist of the caps that we retain could be good.  You can see the whitelist here:

https://github.com/dotcloud/docker/blob/master/daemon/execdriver/native/template/default_template.go