Does the dotnet tool use the global system installed (%AppData%) NuGet.config file though?
You can learn more about NuGet sources from their docs.
Docker is it’s own isolated environment & only has access to the files within it. Whenever you’re installing software and running tasks in your Dockerfile it’s modifying the docker image.
This may be an issue with the latest .NET Core preview, which I think is being used on my builds since I had installed it with Visual Studio 2019 preview:
If you look at the last issue it claims to still be a problem.
This just looks like it’s an issue with private feeds requiring authentication, not about any change in behavior with how NuGet.config is copied.