Add support for DOCKER_* variables in .env file

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

tests/unit/cli/docker_client_test.py

@@ -17,12 +17,12 @@ class DockerClientTestCase(unittest.TestCase):
     def test_docker_client_no_home(self):
         with mock.patch.dict(os.environ):
             del os.environ['HOME']
-            docker_client()
+            docker_client(os.environ)
 
     def test_docker_client_with_custom_timeout(self):
         timeout = 300
         with mock.patch('compose.cli.docker_client.HTTP_TIMEOUT', 300):
-            client = docker_client()
+            client = docker_client(os.environ)
             self.assertEqual(client.timeout, int(timeout))