The image that's being used is mcr.microsoft.com/dotnet/sdk:6.0, which is technically a "fat manifest" that points to a number of platform-specific images. But that's all handled by the Docker engine.
So if you're getting a Windows-based image, then it means the Docker engine is not using Windows. I haven't used DockerDesktop in years, since WSL2 is much more reliable and a similar-to-production experience.
I mentioned this in my other reply, but I would suggest to "play around" with the commands using docker run ... to see if you can get this working using that container.
Once you can get it working from the CLI, then it won't be problem for Buildmaster to do the same thing.
Thanks,
Steve