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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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