Migrate to dotnet repo

[glennc]

See: aspnet/Announcements#298

[felipepessoto]

Are the versions 2.1-sdk and runtime RTM? https://hub.docker.com/r/microsoft/dotnet/tags/

[natemcmaster]

Not yet. These tags currently point to 2.1-preview1. (cc @MichaelSimons @richlander)

[richlander]

Right, preview 1. That is all pretty clear -> https://hub.docker.com/r/microsoft/dotnet/

There will be a preview 2, too.

[natemcmaster]

The work to move these images was done and builds are now available. See https://hub.docker.com/r/microsoft/dotnet-nightly/ to try the new runtime images.

FROM microsoft/dotnet-nightly:2.1-aspnetcore-runtime

I've also removed the code from the dev branch of this repo for producing nightly builds of 2.1-preview2. Once preview2 ships, I'll also remove the code for the 2.1-preview1 images currently in master.