`scale` property should be merged according to standard scalar rules

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

compose/config/config.py

@@ -116,6 +116,7 @@ ALLOWED_KEYS = DOCKER_CONFIG_KEYS + [
     'logging',
     'network_mode',
     'init',
+    'scale',
 ]
 
 DOCKER_VALID_URL_PREFIXES = (

tests/unit/config/config_test.py

@@ -2098,6 +2098,19 @@ class ConfigTest(unittest.TestCase):
         actual = config.merge_service_dicts(base, override, V3_3)
         assert actual['credential_spec'] == override['credential_spec']
 
+    def test_merge_scale(self):
+        base = {
+            'image': 'bar',
+            'scale': 2,
+        }
+
+        override = {
+            'scale': 4,
+        }
+
+        actual = config.merge_service_dicts(base, override, V2_2)
+        assert actual == {'image': 'bar', 'scale': 4}
+
     def test_external_volume_config(self):
         config_details = build_config_details({
             'version': '2',