i am trying to build the .net project.
when i use FROM mcr.microsoft.com/dotnet/sdk:5.0 AS build
i got this error 
so, the project is developed by 4.5 Framework. I can not upgrade (Businnes matter).
so i google it and found this
so i change the syntax into FROM mcr.microsoft.com/dotnet/framework/sdk:4.8 AS build
now i got this 
i am new to docker. not know what the meaning of the error message.
FROM mcr.microsoft.com/dotnet/sdk:5.0 AS build
It means you are going to use .net core 5.0 which should be (preinstalled) in image.
So the image
FROM mcr.microsoft.com/dotnet/framework/sdk:4.8 AS build
will be deployed based on an image with preinstalled .net framework 4.8.
Unfortunately, according to this:
https://hub.docker.com/_/microsoft-dotnet-framework-sdk/
There are versions only for 4.8 and 3.5.
I am pretty sure that update .net framework from 4.5 to 4.8 is possible.
So I would update .net framework version and use docker image for .net framework 4.8 .
