Allow to extend service using shorthand notation. Closes #1989

Signed-off-by: Karol Duleba <mr.fuxi@gmail.com>

tests/fixtures/extends/verbose-and-shorthand.yml

@@ -0,0 +1,15 @@
+base:
+  image: busybox
+  environment:
+    - "BAR=1"
+
+verbose:
+  extends:
+    service: base
+  environment:
+    - "FOO=1"
+
+shorthand:
+  extends: base
+  environment:
+    - "FOO=2"

tests/unit/config/config_test.py

@@ -1115,6 +1115,26 @@ class ExtendsTest(unittest.TestCase):
         dicts = load_from_filename('tests/fixtures/extends/valid-common-config.yml')
         self.assertEqual(dicts[0]['environment'], {'FOO': '1'})
 
+    def test_extended_service_with_verbose_and_shorthand_way(self):
+        services = load_from_filename('tests/fixtures/extends/verbose-and-shorthand.yml')
+        self.assertEqual(service_sort(services), service_sort([
+            {
+                'name': 'base',
+                'image': 'busybox',
+                'environment': {'BAR': '1'},
+            },
+            {
+                'name': 'verbose',
+                'image': 'busybox',
+                'environment': {'BAR': '1', 'FOO': '1'},
+            },
+            {
+                'name': 'shorthand',
+                'image': 'busybox',
+                'environment': {'BAR': '1', 'FOO': '2'},
+            },
+        ]))
+
 
 @pytest.mark.xfail(IS_WINDOWS_PLATFORM, reason='paths use slash')
 class ExpandPathTest(unittest.TestCase):