Handle the 'network_mode' key when merging multiple compose files.

Fixes docker/compose#2840.

Signed-off-by: Lukas Waslowski <[email protected]>

compose/config/config.py

@@ -87,6 +87,7 @@ ALLOWED_KEYS = DOCKER_CONFIG_KEYS + [
     'container_name',
     'dockerfile',
     'logging',
+    'network_mode',
 ]
 
 DOCKER_VALID_URL_PREFIXES = (