tests: Document Python versions in build_docker_images.py

tests/image_prep/build_docker_images.py

@@ -24,9 +24,9 @@ def sh(s, *args):
 label_by_id = {}
 
 for base_image, label in [
-        ('debian:stretch', 'debian'),
-        ('centos:6', 'centos6'),
-        ('centos:7', 'centos7')
+        ('debian:stretch', 'debian'),  # Python 2.7.13, 3.5.3
+        ('centos:6', 'centos6'),       # Python 2.6.6
+        ('centos:7', 'centos7')        # Python 2.7.5
     ]:
     args = sh('docker run --rm -it -d -h mitogen-%s %s /bin/bash',
               label, base_image)