add apparmor to the Dockerfile

![break cache](http://treasure.diylol.com/uploads/post/image/362034/resized_all-the-things-meme-generator-break-all-the-things-31afe9.jpg)


As apparmor is enabled by default, if it's enabled on your host, it has to be working with dind

/cc @tianon @crosbymichael