Fix handling of 'cmd'

nargs='*' made a lot of other parsing fail with strange
errors.

repo2docker/app.py

@@ -152,7 +152,7 @@ class Repo2Docker(Application):
 
         argparser.add_argument(
             'cmd',
-            nargs='*',
+            action='append',
             help='Custom command to run after building container'
         )