Correctly merge the 'services/<service>/networks' key in the case of multiple compose files.

Fixes docker/compose#2839.

Signed-off-by: Lukas Waslowski <cr7pt0gr4ph7@gmail.com>

compose/config/config.py

@@ -698,6 +698,7 @@ def merge_service_dicts(base, override, version):
         'depends_on',
         'expose',
         'external_links',
+        'networks',
         'ports',
         'volumes_from',
     ]: