.NET Framework 4.7 Docker Image Now Available

## 4.7 image is now available on [Docker Hub](https://hub.docker.com/r/microsoft/dotnet-framework/).

### Summary
- The latest tag of the .NET Framework image now points to the 4.7 image.
- The way our images layer is now different (although this shouldn’t affect anyone). Read more on the [design and discussion](https://github.com/dotnet/designs/issues/7).
- Check it out with the [4.7 Docker Image Sample](https://github.com/Microsoft/dotnet-framework-docker-samples/tree/master/dotnetapp-4.7)

### Feedback
Please try it out! General feedback can be logged in the [.NET Framework Docker issues](https://github.com/Microsoft/dotnet-framework-docker/issues).
[Microsoft/dotnet-framework-docker #22 ](https://github.com/Microsoft/dotnet-framework-docker/issues/22) is open for discussion.

This issue announces the .NET Framework 4.7 Docker image and links to Docker Hub, the 4.7 image sample, and related discussions. Start by reviewing those linked resources; the issue does not identify a code change, test, or completion criterion for a contributor.

Written by the indexing model from the issue text.