As I read here, it is changed to:
FROM mcr.microsoft.com/dotnet/aspnet:5.0-buster-slim AS base
FROM mcr.microsoft.com/dotnet/sdk:5.0-buster-slim AS build
It is also mentioned on the docker hub with more info:
As part of the .NET 5.0 release, all .NET Docker images (including
.NET Core 2.1 and 3.1) have transitioned to a new set of Docker
repositories described below. Updates will continue to be made to
supported tags in the old repository locations for backwards
compatibility. Please update any repository references to these new
names. For more information see the .NET 5.0 repository rename
announcement.
EDIT:
Image descriptions:
| Image | Comments |
|---|---|
| mcr.microsoft.com/dotnet/runtime:5.0 | .NET 5 multi-architecture: Supports Linux and Windows Nano Server depending on the Docker host. |
| mcr.microsoft.com/dotnet/aspnet:5.0 | ASP.NET Core 5.0 multi-architecture: Supports Linux and Windows Nano Server depending on the Docker host. The aspnetcore image has a few optimizations for ASP.NET Core. |
| mcr.microsoft.com/dotnet/aspnet:5.0-buster-slim | .NET 5 runtime-only on Linux Debian distro |
| mcr.microsoft.com/dotnet/aspnet:5.0-nanoserver-1809 | .NET 5 runtime-only on Windows Nano Server (Windows Server version 1809) |
Image description reference.