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Basic ARG usage

I’m not winning in trying to build a .NET application in Debug / Release mode conditionally using an argument (dotnet build’s “-c” parameter). The argument is not substituted by Docker

So I’ve distilled my DockerFile down to:

FROM microsoft/dotnet:2.2-aspnetcore-runtime-nanoserver-1809 AS base
ARG my_var=Debug
ENV my_var=$my_var
#RUN dotnet build myproject.csproj -c $my_var -o /app    // dotnet build complains 
                                           // about invalid configuration parameter
ENTRYPOINT echo argument is $my_var 

, and run
docker build -t testing .
docker run -it testing

The output never reflects the argument (Here I’m using the default, ‘Debug’, but passing the --build-arg through during build also does the same thing).

The argument / env is never substituted. The run output is:
argument is $my_var

I’ve tried to remove the ENV-creation and all combinations of inverted commas, ${}, etc. to no avail. What haven’t I grasped??

Docker Version:

Client: Docker Engine - Community
Version: 18.09.2
API version: 1.39
Go version: go1.10.8
Git commit: 6247962
Built: Sun Feb 10 04:12:31 2019
OS/Arch: windows/amd64
Experimental: false

I would love to see this documented somewhere, but apparently this is down to the shell that is being used to run the docker command. In Windows, this means that we need to use


rather than


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