Add support for labels during build

Signed-off-by: Colin Hebert <[email protected]>

compose/config/config_schema_v3.2.json

@@ -72,6 +72,7 @@
                 "context": {"type": "string"},
                 "dockerfile": {"type": "string"},
                 "args": {"$ref": "#/definitions/list_or_dict"},
+                "labels": {"$ref": "#/definitions/list_or_dict"},
                 "cache_from": {"$ref": "#/definitions/list_of_strings"}
               },
               "additionalProperties": false