Introduce non-core repositories for .NET 5

[MichaelSimons]

The Introducing .NET 5 blog announces a single unified .NET platform going forward.

.NET 5 = .NET Core vNext

With the .NET 5 release the core terminology is dropped from the naming. As a result it would make sense to do the same in the .NET docker repositories going forward.

Current Repositories:

• dotnet/core: .NET Core -> product family repo
• dotnet/core/sdk: .NET Core SDK
• dotnet/core/aspnet: ASP.NET Core Runtime
• dotnet/core/runtime-deps: .NET Core Runtime Dependencies
• dotnet/core/samples: .NET Core Samples
• dotnet/core-nightly: .NET Core (Preview)

Proposed new repositories for .NET 5 going forward:

• dotnet
• dotnet/sdk
• dotnet/aspnet
• dotnet/runtime-deps
• dotnet/samples
• dotnet-nightly

Benefits:

1. A shorter name is more convenient to type.
2. It will be beneficial to align the Docker repository names with the official product name. Having core in the name won't necessarily be confusing now but over time it will become more confusing especially for new users.

Sample Usage

FROM mcr.microsoft.com/dotnet/sdk:5.0

FROM mcr.microsoft.com/dotnet/runtime:5.0