dockerTools: use more familiar terminology to describe test image

import ./make-test-python.nix ({ pkgs, ... }:

let

  # nixpkgs#214434: dockerTools.buildImage fails to unpack base images

  # containing duplicate rootfs diffs when those duplicate tarballs

  # containing duplicate layers when those duplicate tarballs

  # appear under the manifest's 'Layers'. Docker can generate images

  # like this even though dockerTools does not.

  repeatedLayerTestImage =