Adding custom nuget package source to template

Hey, I want to add a new nuget package source to my .NET8 template (following this: https://github.com/dotnet/dotnet-docker/blob/main/documentation/scenarios/nuget-credentials.md) but when I try to create a new file in my template, I get the following error (see picture). Is there a way that I can add a private nuget source securely to an image?