microsoft/dotnet latest tag updated to .NET Core 2.0 SDK

# microsoft/dotnet `latest` tag updated to .NET Core 2.0 SDK

The `latest` tag for the [microsoft/dotnet](https://hub.docker.com/r/microsoft/dotnet/) repo has been updated to reference the [.NET Core 2.0](https://blogs.msdn.microsoft.com/dotnet/2017/08/14/announcing-net-core-2-0) SDK.

## Discussion

You can share your thoughts on this change or see what others are saying at:

* [dotnet/dotnet-docker #278](https://github.com/dotnet/dotnet-docker/issues/278)

## Details

* [dotnet-docker #277](https://github.com/dotnet/dotnet-docker/pull/277)
* [aspnet-docker #278](https://github.com/aspnet/aspnet-docker/pull/278)

The `latest` tag for the [microsoft/dotnet](https://hub.docker.com/r/microsoft/dotnet/) repo is always intended to reference the latest supported .NET Core SDK. At this time, the latest support SDK is .NET Core SDK 2.0.

The following repos have been updated in the same way:

* [microsoft/dotnet](https://hub.docker.com/r/microsoft/dotnet/)
* [microsoft/aspnetcore](https://hub.docker.com/r/microsoft/aspnetcore/)
* [microsoft/aspnetcore-build/](https://hub.docker.com/r/microsoft/aspnetcore-build/)
* [microsoft/dotnet-samples/](https://hub.docker.com/r/microsoft/dotnet-samples/)

It is recommended to use a more-specific tag than `latest` more anything other than casual development. Once you have an application that you believe will eventually go to production, you are recommended to use a tag like `2.0-sdk`. The [.NET Core Docker Samples](https://github.com/dotnet/dotnet-docker-samples) contains examples of more specific usage such as the [.NET Core Development Sample Dockerfile](https://github.com/dotnet/dotnet-docker-samples/blob/master/dotnetapp-dev/Dockerfile).