Reads list of dataset images from the datastore.

def _read_image_list(self, skip_image_ids=None):
    """Reads list of dataset images from the datastore."""
    if skip_image_ids is None:
      skip_image_ids = []
    images = self._storage_client.list_blobs(
        prefix=os.path.join('dataset', self._dataset_name) + '/')
    zip_files = [i for i in images if i.endswith('.zip')]
    if len(zip_files) == 1:
      # we have a zip archive with images
      zip_name = zip_files[0]
      logging.info('Reading list of images from zip file %s', zip_name)
      blob = self._storage_client.get_blob(zip_name)
      buf = BytesIO()
      logging.info('Downloading zip')
      blob.download_to_file(buf)
      buf.seek(0)
      logging.info('Reading content of the zip')
      with zipfile.ZipFile(buf) as f:
        images = [os.path.join(zip_name, os.path.basename(n))
                  for n in f.namelist() if n.endswith('.png')]
      buf.close()
      logging.info('Found %d images', len(images))
    else:
      # we have just a directory with images, filter non-PNG files
      logging.info('Reading list of images from png files in storage')
      images = [i for i in images if i.endswith('.png')]
      logging.info('Found %d images', len(images))
    # filter images which should be skipped
    images = [i for i in images
              if os.path.basename(i)[:-4] not in skip_image_ids]
    # assign IDs to images
    images = [(DATASET_IMAGE_ID_PATTERN.format(idx), i)
              for idx, i in enumerate(sorted(images))]
    return images