Added Dockerfile to download the nightly build of ASP.NET 5

When a developer would like to run against the latest build of ASP.NET 5 he can use the `microsoft/aspnet:nightly` image. This image will point to the `https://www.myget.org/F/aspnetvnext/api/v2` feed to get the runtime and us an ONBUILD to get the latest version of KRE.

@giggio has brought this improvement in issue: [Create container for aspnet latest dev version](https://github.com/aspnet/aspnet-docker/issues/10)