Docker multistage build doesn't pass arguments to second stage

Question

I have dockerbuild file that contains from 2 stages

ARG DOTNET_VERSION=net48
ARG CONFIGURATION=Release
FROM mcr.microsoft.com/dotnet/framework/sdk:4.8 AS build-env
ARG DOTNET_VERSION
ARG CONFIGURATION

RUN echo .Net version: $env:DOTNET_VERSION

FROM mcr.microsoft.com/windows/nanoserver:1809
ARG DOTNET_VERSION

RUN echo .Net version: $env:DOTNET_VERSION

I start it using next command: docker build -t test . --build-arg DOTNET_VERSION=net471 --no-cache

Output is:

Step 1/9 : ARG DOTNET_VERSION=net48
Step 2/9 : ARG CONFIGURATION=Release
Step 3/9 : FROM mcr.microsoft.com/dotnet/framework/sdk:4.8 AS build-env
 ---> c1e2ba0a9132
Step 4/9 : ARG DOTNET_VERSION
 ---> Running in 35ac4620bae3
Removing intermediate container 35ac4620bae3
 ---> 2000301c9b49
Step 5/9 : ARG CONFIGURATION
 ---> Running in 06b7c39a01b2
Removing intermediate container 06b7c39a01b2
 ---> 1591528894c8
Step 6/9 : RUN echo .Net version: $env:DOTNET_VERSION
 ---> Running in 6692bbe08eae
.Net
version:
net471
Removing intermediate container 6692bbe08eae
 ---> 7c4fa49a7732
Step 7/9 : FROM mcr.microsoft.com/windows/nanoserver:1809
 ---> 8572826a0d1a
Step 8/9 : ARG DOTNET_VERSION
 ---> Running in 3a1d9f759aaa
Removing intermediate container 3a1d9f759aaa
 ---> 74a6ccf7960f
Step 9/9 : RUN echo .Net version: $env:DOTNET_VERSION
 ---> Running in 13122b910b39
.Net version: $env:DOTNET_VERSION
Removing intermediate container 13122b910b39
 ---> 1c5b754b0b56
Successfully built 1c5b754b0b56

Why step 6 and step 9 results are different?

Answer 1

I found the problem

If mcr.microsoft.com/windows/nanoserver:1809 image is used then arguments should be used in %arg% format.

If mcr.microsoft.com/dotnet/framework/sdk:4.8 image is used then arguments should be used in $env:arg format.

It is confusing and I haven't found where it is documented.