Improve merging for several service config attributes

All uniqueItems lists in the config now receive the same treatment removing
duplicates.

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

compose/config/config.py

@@ -4,7 +4,6 @@ from __future__ import unicode_literals
 import functools
 import logging
 import ntpath
-import operator
 import os
 import string
 import sys
@@ -744,18 +743,15 @@ def merge_service_dicts(base, override, version):
     md.merge_mapping('ulimits', parse_ulimits)
     md.merge_mapping('networks', parse_networks)
     md.merge_sequence('links', ServiceLink.parse)
-    md.merge_field('ports', merge_port_mappings, default=[])
 
     for field in ['volumes', 'devices']:
         md.merge_field(field, merge_path_mappings)
 
     for field in [
-        'depends_on',
-        'expose',
-        'external_links',
-        'volumes_from',
+        'ports', 'cap_add', 'cap_drop', 'expose', 'external_links',
+        'security_opt', 'volumes_from', 'depends_on',
     ]:
-        md.merge_field(field, operator.add, default=[])
+        md.merge_field(field, merge_unique_items_lists, default=[])
 
     for field in ['dns', 'dns_search', 'env_file', 'tmpfs']:
         md.merge_field(field, merge_list_or_string)
@@ -771,8 +767,8 @@ def merge_service_dicts(base, override, version):
     return dict(md)
 
 
-def merge_port_mappings(base, override):
-    return list(set().union(base, override))
+def merge_unique_items_lists(base, override):
+    return sorted(set().union(base, override))
 
 
 def merge_build(output, base, override):