docs: def image_name(self):
          """
          The image_name of a container is the concatenation of the ``image_index``,
          ``image_name_prefix``, and ``name`` of the image.
  
          Also, if $EXTRA_IMAGE_NAME is defined, that is appended
          """
          if getattr(self, "_image_name", NotSpecified) is NotSpecified:
              self._image_name = self.prefixed_image_name
  
              if self.image_index:
                  self._image_name = "{0}{1}".format(self.image_index, self._image_name)
  
              if "EXTRA_IMAGE_NAME" in os.environ:
                  self._image_name = "{0}{1}".format(self._image_name, os.environ["EXTRA_IMAGE_NAME"])
          return self._image_name