Added default env file test.

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

tests/fixtures/default-env-file/.env

@@ -0,0 +1,4 @@
+IMAGE=alpine:latest
+COMMAND=true
+PORT1=5643
+PORT2=9999

tests/fixtures/default-env-file/docker-compose.yml

@@ -0,0 +1,6 @@
+web:
+    image: ${IMAGE}
+    command: ${COMMAND}
+    ports:
+        - $PORT1
+        - $PORT2

tests/unit/config/config_test.py

@@ -1582,6 +1582,19 @@ class PortsTest(unittest.TestCase):
 
 
 class InterpolationTest(unittest.TestCase):
+    @mock.patch.dict(os.environ)
+    def test_config_file_with_environment_file(self):
+        service_dicts = config.load(
+            config.find('tests/fixtures/default-env-file', None)
+        ).services
+
+        self.assertEqual(service_dicts[0], {
+            'name': 'web',
+            'image': 'alpine:latest',
+            'ports': ['5643', '9999'],
+            'command': 'true'
+        })
+
     @mock.patch.dict(os.environ)
     def test_config_file_with_environment_variable(self):
         os.environ.update(