Assorted updates to Dockerfile

[friism]

• use multistage builds
• use overlay networking
• use nightly images

[natemcmaster]

Thanks for trying to solve this. However, after discussing with the team, we've decided to remove Dockerfiles from this repo altogether. We don't use these files in our testing anymore, which is why they are broken and stale.

FYI - this repository is actually meant to be a test application, and isn't really supposed to be a sample or tutorial app. There are a handful of documents on https://docs.asp.net that demonstrate how to use ASP.NET Core with Docker. The source code for our docs is also open source and takes contributions. See https://github.com/aspnet/Docs