Allow user=0 to be passed in create_container

Signed-off-by: Anthony Sottile <asottile@umich.edu>

docker/types/containers.py

@@ -578,7 +578,7 @@ class ContainerConfig(dict):
             'Hostname': hostname,
             'Domainname': domainname,
             'ExposedPorts': ports,
-            'User': six.text_type(user) if user else None,
+            'User': six.text_type(user) if user is not None else None,
             'Tty': tty,
             'OpenStdin': stdin_open,
             'StdinOnce': stdin_once,

tests/unit/types_containers_test.py

+from docker.types.containers import ContainerConfig
+
+
+def test_uid_0_is_not_elided():
+    x = ContainerConfig(image='i', version='v', command='true', user=0)
+    assert x['User'] == '0'