Typically I use following images for .net core 3.1 and it works fine.
FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim AS base
FROM mcr.microsoft.com/dot
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.