Do not add mode in volume representation if it's not a host binding

Signed-off-by: Joffrey F <joffrey@docker.com>

compose/config/types.py

@@ -203,7 +203,8 @@ class VolumeSpec(namedtuple('_VolumeSpec', 'external internal mode')):
 
     def repr(self):
         external = self.external + ':' if self.external else ''
-        return '{ext}{v.internal}:{v.mode}'.format(ext=external, v=self)
+        mode = ':' + self.mode if self.external else ''
+        return '{ext}{v.internal}{mode}'.format(mode=mode, ext=external, v=self)
 
     @property
     def is_named_volume(self):

tests/acceptance/cli_test.py

@@ -299,7 +299,7 @@ class CLITestCase(DockerClientTestCase):
                 },
                 'volume': {
                     'image': 'busybox',
-                    'volumes': ['/data:rw'],
+                    'volumes': ['/data'],
                     'network_mode': 'bridge',
                 },
                 'app': {