GenerateDockerfilesCommand fails when there isn't a previously generated Dockerfile

**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.