.NET Core 2.1 container images will be deleted from Docker Hub

# .NET Core 2.1 container images will be deleted from Docker Hub

Starting on August 21st, .NET Core 2.1 Docker container images will no longer be available on Docker Hub, but exclusively on Microsoft Container Registry (MCR). This change was previously announced with https://github.com/dotnet/dotnet-docker/issues/2848. If you are reliant on .NET Core 2.1 images on Docker Hub, you should switch to using MCR immediately. Please reach out at dotnet@microsoft.com if this change is a problem for you / your organization.

.NET Blog: https://devblogs.microsoft.com/dotnet/net-core-2-1-container-images-will-be-deleted-from-docker-hub/

## Discussion
Please share your thoughts on this change and see what others are saying at:

[dotnet/dotnet-docker #3052](https://github.com/dotnet/dotnet-docker/issues/3052)

## Details

[.NET images](https://hub.docker.com/_/microsoft-dotnet) are available on MCR from the following repos:

- `mcr.microsoft.com/dotnet/runtime-deps`
- `mcr.microsoft.com/dotnet/runtime`
- `mcr.microsoft.com/dotnet/aspnet`
- `mcr.microsoft.com/dotnet/sdk`

You need to make the following transformations to pull .NET Core 2.1 images from MCR instead of Docker Hub.

For the SDK:

```Dockerfile
microsoft/dotnet:2.1-sdk -> mcr.microsoft.com/dotnet/sdk:2.1
microsoft/dotnet:2-sdk -> mcr.microsoft.com/dotnet/sdk:2.1
microsoft/dotnet:2.1-sdk-stretch -> mcr.microsoft.com/dotnet/sdk:2.1-stretch
microsoft/dotnet:2.1-sdk-stretch-arm32v7 -> mcr.microsoft.com/dotnet/sdk:2.1-stretch-arm32v7
microsoft/dotnet:2.1-sdk-nanoserver-1809 -> mcr.microsoft.com/dotnet/sdk:2.1-nanoserver-1809
microsoft/dotnet:2.1-sdk-alpine -> mcr.microsoft.com/dotnet/sdk:2.1-alpine
microsoft/dotnet:2.1-sdk-bionic -> mcr.microsoft.com/dotnet/sdk:2.1-bionic
microsoft/dotnet:2.1-sdk-bionic-arm32v7 -> mcr.microsoft.com/dotnet/sdk:2.1-bionic-arm32v7
microsoft/dotnet:latest -> mcr.microsoft.com/dotnet/sdk:2.1
```

For the ASP.NET Core:

```Dockerfile
microsoft/dotnet:2.1-aspnetcore-runtime -> mcr.microsoft.com/dotnet/aspnet:2.1
microsoft/dotnet:2-aspnetcore-runtime-> mcr.microsoft.com/dotnet/aspnet:2.1
microsoft/dotnet:2.1-aspnetcore-runtime-stretch-slim -> mcr.microsoft.com/dotnet/aspnet:2.1-stretch-slim
microsoft/dotnet:2.1-aspnetcore-runtime-stretch-slim-arm32v7 -> mcr.microsoft.com/dotnet/aspnet:2.1-stretch-slim-arm32v7
microsoft/dotnet:2.1-aspnetcore-runtime-nanoserver-1809 -> mcr.microsoft.com/dotnet/aspnet:2.1-nanoserver-1809
microsoft/dotnet:2.1-aspnetcore-runtime-alpine -> mcr.microsoft.com/dotnet/aspnet:2.1-alpine
microsoft/dotnet:2.1-aspnetcore-runtime-bionic -> mcr.microsoft.com/dotnet/aspnet:2.1-bionic
microsoft/dotnet:2.1-aspnetcore-runtime-bionic-arm32v7 -> mcr.microsoft.com/dotnet/aspnet:2.1-bionic-arm32v7
microsoft/dotnet:aspnetcore-runtime-> mcr.microsoft.com/dotnet/aspnet:2.1
```

For .NET Runtime:

```Dockerfile
microsoft/dotnet:2.1-runtime -> mcr.microsoft.com/dotnet/runtime:2.1
microsoft/dotnet:2-runtime -> mcr.microsoft.com/dotnet/runtime:2.1
microsoft/dotnet:2.1-runtime-stretch-slim -> mcr.microsoft.com/dotnet/runtime:2.1-stretch-slim
microsoft/dotnet:2.1-runtime-stretch-slim-arm32v7 -> mcr.microsoft.com/dotnet/runtime:2.1-stretch-slim-arm32v7
microsoft/dotnet:2.1-runtime-nanoserver-1809 -> mcr.microsoft.com/dotnet/runtime:2.1-nanoserver-1809
microsoft/dotnet:2.1-runtime-alpine -> mcr.microsoft.com/dotnet/runtime:2.1-alpine
microsoft/dotnet:2.1-runtime-bionic -> mcr.microsoft.com/dotnet/runtime:2.1-bionic
microsoft/dotnet:2.1-runtime-bionic-arm32v7 -> mcr.microsoft.com/dotnet/runtime:2.1-bionic-arm32v7
microsoft/dotnet:runtime -> mcr.microsoft.com/dotnet/runtime:2.1
```

For .NET Runtime dependencies:

```Dockerfile
microsoft/dotnet:2.1-runtime-deps -> mcr.microsoft.com/dotnet/runtime-deps:2.1
microsoft/dotnet:2.1-runtime-deps-stretch-slim -> mcr.microsoft.com/dotnet/runtime-deps:2.1-stretch-slim
microsoft/dotnet:2.1-runtime-deps-stretch-slim-arm32v7 -> mcr.microsoft.com/dotnet/runtime-deps:2.1-stretch-slim-arm32v7
microsoft/dotnet:2.1-runtime-deps-alpine -> mcr.microsoft.com/dotnet/runtime-deps:2.1-alpine
microsoft/dotnet:2.1-runtime-deps-bionic -> mcr.microsoft.com/dotnet/runtime-deps:2.1-bionic
microsoft/dotnet:2.1-runtime-deps-bionic-arm32v7 -> mcr.microsoft.com/dotnet/runtime-deps:2.1-bionic-arm32v7
microsoft/dotnet:runtime-deps -> mcr.microsoft.com/dotnet/runtime-deps:2.1
```