Update of VS Tools for Docker doc (#7772)
Fixes https://github.com/aspnet/Docs/issues/6813
Fixes https://github.com/aspnet/Docs/issues/6128
Fixes https://github.com/aspnet/Docs/issues/5631
**Internal Review Pages**
* [2.1](https://review.docs.microsoft.com/en-us/aspnet/core/host-and-deploy/docker/visual-studio-tools-for-docker?view=aspnetcore-2.1&branch=pr-en-us-7772)
* [2.0](https://review.docs.microsoft.com/en-us/aspnet/core/host-and-deploy/docker/visual-studio-tools-for-docker?view=aspnetcore-2.0&branch=pr-en-us-7772)
