code: func FullPath(remote builder.Source, path string) (string, error) {
  	fullPath, err := remote.Root().ResolveScopedPath(path, true)
  	if err != nil {
  		if runtime.GOOS == "windows" {
  			return "", fmt.Errorf("failed to resolve scoped path %s (%s): %s. Possible cause is a forbidden path outside the build context", path, fullPath, err)
  		}
  		return "", fmt.Errorf("Forbidden path outside the build context: %s (%s)", path, fullPath) // backwards compat with old error
  	}
  	return fullPath, nil
  }
docs: // FullPath is a helper for getting a full path for a path from a source