Update ASP.NET docker container to new URI

[Rodney Richardson]

https://hub.docker.com/_/microsoft-dotnet-aspnet

Microsoft have moved/renamed their docker containers as part of .NET Core 5,0 release.

You should be able to rename the ASP.NET docker container used to the LTS version without any other changes: mcr.microsoft.com/dotnet/aspnet:3.1

[Federico Barresi]

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