Support 'buildargs' parameter for docker 1.9.

Signed-off-by: Leonty Balovnev <tigreavecdesailes@gmail.com>

docker/api/build.py

 import logging
 import os
 import re
+import json
 
 from .. import constants
 from .. import errors
@@ -16,7 +17,7 @@ class BuildApiMixin(object):
               nocache=False, rm=False, stream=False, timeout=None,
               custom_context=False, encoding=None, pull=False,
               forcerm=False, dockerfile=None, container_limits=None,
-              decode=False):
+              decode=False, buildargs=None):
         remote = context = headers = None
         container_limits = container_limits or {}
         if path is None and fileobj is None:
@@ -71,6 +72,14 @@ class BuildApiMixin(object):
         }
         params.update(container_limits)
 
+        if buildargs:
+            if utils.version_gte(self._version, '1.21'):
+                params.update({'buildargs': json.dumps(buildargs)})
+            else:
+                raise errors.InvalidVersion(
+                    'buildargs was only introduced in API version 1.21'
+                )
+
         if context is not None:
             headers = {'Content-Type': 'application/tar'}
             if encoding:

tests/integration/build_test.py

@@ -96,3 +96,18 @@ class BuildTest(api_test.BaseTestCase):
             list(filter(None, logs.split('\n'))),
             ['not-ignored'],
         )
+
+    @requires_api_version('1.21')
+    def test_build_with_buildargs(self):
+        script = io.BytesIO('\n'.join([
+            'FROM scratch',
+            'ARG test',
+            'USER $test'
+        ]).encode('ascii'))
+
+        self.client.build(fileobj=script, tag='buildargs',
+                          buildargs={'test': 'OK'}, stream=True)
+        self.tmp_imgs.append('buildargs')
+
+        info = self.client.inspect_image('buildargs')
+        self.assertEqual(info['Config']['User'], 'OK')