Provide valid serialization of depends_on when format is not 2.1

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

compose/config/serialize.py

@@ -56,9 +56,16 @@ def denormalize_service_dict(service_dict, version):
     service_dict = service_dict.copy()
 
     if 'restart' in service_dict:
-        service_dict['restart'] = types.serialize_restart_spec(service_dict['restart'])
+        service_dict['restart'] = types.serialize_restart_spec(
+            service_dict['restart']
+        )
 
     if version == V1 and 'network_mode' not in service_dict:
         service_dict['network_mode'] = 'bridge'
 
+    if 'depends_on' in service_dict and version != V2_1:
+        service_dict['depends_on'] = sorted([
+            svc for svc in service_dict['depends_on'].keys()
+        ])
+
     return service_dict

tests/unit/config/config_test.py

@@ -22,6 +22,7 @@ from compose.config.config import V3_0
 from compose.config.environment import Environment
 from compose.config.errors import ConfigurationError
 from compose.config.errors import VERSION_EXPLANATION
+from compose.config.serialize import denormalize_service_dict
 from compose.config.types import VolumeSpec
 from compose.const import IS_WINDOWS_PLATFORM
 from compose.utils import nanoseconds_from_time_seconds
@@ -3269,3 +3270,33 @@ def get_config_filename_for_files(filenames, subdir=None):
         return os.path.basename(filename)
     finally:
         shutil.rmtree(project_dir)
+
+
+class SerializeTest(unittest.TestCase):
+    def test_denormalize_depends_on_v3(self):
+        service_dict = {
+            'image': 'busybox',
+            'command': 'true',
+            'depends_on': {
+                'service2': {'condition': 'service_started'},
+                'service3': {'condition': 'service_started'},
+            }
+        }
+
+        assert denormalize_service_dict(service_dict, V3_0) == {
+            'image': 'busybox',
+            'command': 'true',
+            'depends_on': ['service2', 'service3']
+        }
+
+    def test_denormalize_depends_on_v2_1(self):
+        service_dict = {
+            'image': 'busybox',
+            'command': 'true',
+            'depends_on': {
+                'service2': {'condition': 'service_started'},
+                'service3': {'condition': 'service_started'},
+            }
+        }
+
+        assert denormalize_service_dict(service_dict, V2_1) == service_dict