docs: def external_dependencies(self):
          """
          Return all the external images this Dockerfile will depend on
  
          These are images from self.dependent_images that aren't defined in this configuration.
          """
          found = []
          for dep in self.dependent_images:
              if isinstance(dep, six.string_types):
                  if dep not in found:
                      yield dep
                      found.append(dep)