Merge extra_hosts instead of overwrite

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

compose/config/config.py

@@ -717,9 +717,6 @@ def process_service(service_config):
     if 'labels' in service_dict:
         service_dict['labels'] = parse_labels(service_dict['labels'])
 
-    if 'extra_hosts' in service_dict:
-        service_dict['extra_hosts'] = parse_extra_hosts(service_dict['extra_hosts'])
-
     if 'sysctls' in service_dict:
         service_dict['sysctls'] = build_string_dict(parse_sysctls(service_dict['sysctls']))
 
@@ -947,6 +944,7 @@ def merge_service_dicts(base, override, version):
     md.merge_sequence('secrets', types.ServiceSecret.parse)
     md.merge_sequence('configs', types.ServiceConfig.parse)
     md.merge_mapping('deploy', parse_deploy)
+    md.merge_mapping('extra_hosts', parse_extra_hosts)
 
     for field in ['volumes', 'devices']:
         md.merge_field(field, merge_path_mappings)

tests/unit/config/config_test.py

@@ -2209,6 +2209,24 @@ class ConfigTest(unittest.TestCase):
             }
         }
 
+    def test_merge_extra_hosts(self):
+        base = {
+            'image': 'bar',
+            'extra_hosts': {
+                'foo': '1.2.3.4',
+            }
+        }
+
+        override = {
+            'extra_hosts': ['bar:5.6.7.8', 'foo:127.0.0.1']
+        }
+
+        actual = config.merge_service_dicts(base, override, V2_0)
+        assert actual['extra_hosts'] == {
+            'foo': '127.0.0.1',
+            'bar': '5.6.7.8',
+        }
+
     def test_merge_healthcheck_config(self):
         base = {
             'image': 'bar',