.NET Core 2.0 multi-arch tags now use Debian Stretch

# .NET Core 2.0 multi-arch tags now use Debian Stretch

## Summary

[.NET Core 2.0 Docker multi-arch tags](https://hub.docker.com/r/microsoft/dotnet/) now use [Debian Stretch](https://www.debian.org/releases/stretch/) instead of [Debian Jessie](https://www.debian.org/releases/jessie/). .NET Core 2.0 Debian Jessie tags will still be provided. Debian Stretch is considered by the .NET Core team to be a better default choice for .NET Core Docker Linux image users for the lifetime of .NET Core 2.x. There are no plans to update .NET Core 1.x in the same way.

## Details

- [Debian 9 Stretch released](https://www.debian.org/News/2017/20170617)
- [Change 2.0 multi-arch tags to be based on stretch -- dotnet/dotnet-docker-nightly #322](https://github.com/dotnet/dotnet-docker-nightly/pull/322)
- [.NET Core Docker images will move to multi-arch based tags -- dotnet/announcements #14](https://github.com/dotnet/announcements/issues/14)

## Impact

The impact of this change should be minimal. It is possible that you may notice differences in Debian Stretch relative to Debian Jessie. This change would be considered a breaking change if it was made for existing 1.x multi-arch tags. It is considered acceptable because it is being made between major .NET versions (during .NET Core 2.0 preview phase).

The `latest` tag will be switched to the .NET Core 2.0 SDK when .NET Core 2.0 ships as RTM. This change will result in `latest` changing from Debian Jessie to Debian Stretch.

This issue is an announcement about .NET Core 2.0 Docker multi-arch tags moving from Debian Jessie to Stretch. Start with the linked Docker Hub images and referenced dotnet/dotnet-docker-nightly pull request; done would require a specific repository file or documentation task, which this issue does not identify.

Written by the indexing model from the issue text.