Add support for experimental platform flag in build and pull

[shin-]

Fixes #1855

[feliperuhland]

Hi @shin-. I think you just need to fix a test:

diff --git a/tests/unit/models_containers_test.py b/tests/unit/models_containers_test.py
index a479e83..95295a9 100644
--- a/tests/unit/models_containers_test.py
+++ b/tests/unit/models_containers_test.py
@@ -225,7 +225,7 @@ class ContainerCollectionTest(unittest.TestCase):
         container = client.containers.run('alpine', 'sleep 300', detach=True)
 
         assert container.id == FAKE_CONTAINER_ID
-        client.api.pull.assert_called_with('alpine', tag=None)
+        client.api.pull.assert_called_with('alpine', platform=None, tag=None)
 
     def test_run_with_error(self):
         client = make_fake_client()

Jenkins:

=================================== FAILURES ===================================
____________________ ContainerCollectionTest.test_run_pull _____________________
tests/unit/models_containers_test.py:228: in test_run_pull
    client.api.pull.assert_called_with('alpine', tag=None)
/opt/python/3.5.4/lib/python3.5/unittest/mock.py:794: in assert_called_with
    raise AssertionError(_error_message()) from cause
E   AssertionError: Expected call: pull('alpine', tag=None)
E   Actual call: pull('alpine', platform=None, tag=None)
========== 1 failed, 500 passed, 4 skipped, 1 xfailed in 4.73 seconds ==========

[shin-]

@feliperuhland Thanks!