Add support for user on exec_create.

docker/client.py

@@ -521,19 +521,26 @@ class Client(requests.Session):
         return self.exec_start(create_res, detach, tty, stream)
 
     def exec_create(self, container, cmd, stdout=True, stderr=True, tty=False,
-                    privileged=False):
+                    user=None, privileged=False):
         if utils.compare_version('1.15', self._version) < 0:
             raise errors.InvalidVersion('Exec is not supported in API < 1.15')
         if privileged and utils.compare_version('1.19', self._version) < 0:
             raise errors.InvalidVersion(
                 'Privileged exec is not supported in API < 1.19'
             )
+        if user and utils.compare_version('1.19', self._version) < 0:
+            raise errors.InvalidVersion(
+                'User-specific exec is not supported in API < 1.19'
+            )
         if isinstance(cmd, six.string_types):
             cmd = shlex.split(str(cmd))
 
+        if user is None:
+            user = ''
+
         data = {
             'Container': container,
-            'User': '',
+            'User': user,
             'Privileged': privileged,
             'Tty': tty,
             'AttachStdin': False,

tests/integration_test.py

@@ -829,6 +829,23 @@ class TestExecuteCommandString(BaseTestCase):
         self.assertEqual(exec_log, expected)
 
 
+@unittest.skipIf(not EXEC_DRIVER_IS_NATIVE, 'Exec driver not native')
+class TestExecuteCommandStringAsUser(BaseTestCase):
+    def runTest(self):
+        container = self.client.create_container('busybox', 'cat',
+                                                 detach=True, stdin_open=True)
+        id = container['Id']
+        self.client.start(id)
+        self.tmp_containers.append(id)
+
+        res = self.client.exec_create(id, 'whoami', user='default')
+        self.assertIn('Id', res)
+
+        exec_log = self.client.exec_start(res)
+        expected = b'default' if six.PY3 else 'default\n'
+        self.assertEqual(exec_log, expected)
+
+
 @unittest.skipIf(not EXEC_DRIVER_IS_NATIVE, 'Exec driver not native')
 class TestExecuteCommandStreaming(BaseTestCase):
     def runTest(self):