Unset env vars behavior in 'run' mirroring engine

Unset env vars passed to `run` via command line options take the
value of the system's var with the same name.

Signed-off-by: Joffrey F <joffrey@docker.com>

tests/acceptance/cli_test.py

@@ -1216,6 +1216,14 @@ class CLITestCase(DockerClientTestCase):
             'simplecomposefile_simple_run_1',
             'exited'))
 
+    @mock.patch.dict(os.environ)
+    def test_run_env_values_from_system(self):
+        os.environ['FOO'] = 'bar'
+        os.environ['BAR'] = 'baz'
+        result = self.dispatch(['run', '-e', 'FOO', 'simple', 'env'], None)
+        assert 'FOO=bar' in result.stdout
+        assert 'BAR=baz' not in result.stdout
+
     def test_rm(self):
         service = self.project.get_service('simple')
         service.create_container()