Added onbuild Dockerfile and Kestrel fix

First of all, great to see that Microsoft helps to run ASP.NET 5 application on Docker. This opens up a ton of possibilities. The last days I was creating images on my own to test out ASP.NET vNext on CoreOS, but this make things a bit easier.

Personally when I deploy an application using a Docker container I like doing this using a `onbuild` version of the Docker image. An example of using this `onbuild` image could be found [here](https://gist.github.com/vlesierse/b806f430d782975c8852). This is not something new and you see that for instance [Node](https://github.com/docker-library/node/blob/806873187edbff36d7fc294a62131f4f28fa681a/0.10/onbuild/Dockerfile) is also doing this.

Next thing this pull request addresses is the fact the Kestrel isn't running on Linux at the moment. There is already a [issue](https://github.com/aspnet/KestrelHttpServer/issues/10) reported, but it could help people getting the images and there code working correctly.

This pull request isn't really to contribute the code directly but start the discussion and ask for feedback.

PS. I love the direction Microsoft is moving to.