address PR feedback

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

compose/service.py

@@ -6,6 +6,7 @@ import os
 import re
 import sys
 from collections import namedtuple
+from operator import attrgetter
 
 import enum
 import six
@@ -1009,10 +1010,6 @@ def parse_volume_from_spec(volume_from_config):
     else:
         source, mode = parts
 
-    if mode not in ('rw', 'ro'):
-        raise ConfigError("VolumeFrom %s has invalid mode (%s), should be "
-                          "one of: rw, ro." % (volume_from_config, mode))
-
     return VolumeFromSpec(source, mode)