GenerateDockerfilesCommand fails when there isn't a previously generated Dockerfile

[MichaelSimons]

Example failure:

System.IO.FileNotFoundException: Path specified in manifest file does not exist.
File name: 'src/aspnet/5.0/nanoserver-2004/amd64/Dockerfile'
   at Microsoft.DotNet.ImageBuilder.ViewModel.ModelExtensions.ValidateFileReference(String path, String manifestDirectory) in /image-builder/src/ViewModel/ModelExtensions.cs:line 98
   at Microsoft.DotNet.ImageBuilder.ViewModel.ModelExtensions.ValidateImage(Image image, String manifestDirectory) in /image-builder/src/ViewModel/ModelExtensions.cs:line 132
   at Microsoft.DotNet.ImageBuilder.ViewModel.ModelExtensions.ValidateRepo(Repo repo, String manifestDirectory) in /image-builder/src/ViewModel/ModelExtensions.cs:line 126
   at Microsoft.DotNet.ImageBuilder.ViewModel.ModelExtensions.Validate(Manifest manifest, String manifestDirectory) in /image-builder/src/ViewModel/ModelExtensions.cs:line 63
   at Microsoft.DotNet.ImageBuilder.ViewModel.ManifestInfo.Create(String manifestPath, ManifestFilter manifestFilter, IManifestOptionsInfo options) in /image-builder/src/ViewModel/ManifestInfo.cs:line 64
   at Microsoft.DotNet.ImageBuilder.ViewModel.ManifestInfo.Load(IManifestOptionsInfo options) in /image-builder/src/ViewModel/ManifestInfo.cs:line 45
   at Microsoft.DotNet.ImageBuilder.Commands.ManifestCommand`1.LoadManifest() in /image-builder/src/Commands/ManifestCommand.cs:line 16
   at Microsoft.DotNet.ImageBuilder.ImageBuilder.Main(String[] args) in /image-builder/src/ImageBuilder.cs:line 66

The manifest validation logic is getting in the way for this scenario. This logic runs when the manifest is loaded and validates that every path in the manifest exists. You should be able to use the GenerateDockerfilesCommand to generated the Dockerfiles instead of having to stub them out first.

[lbussell]

[Triage] Related: https://github.com/dotnet/dotnet-docker/issues/4704