compose: add build.extra_hosts to v3.9 schema

This is not currently used by the CLI, but can be used by
docker compose to bring parity on this feature with the
compose v2.4 schema.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/compose/schema/data/config_schema_v3.9.json

@@ -88,7 +88,8 @@
                 "cache_from": {"$ref": "#/definitions/list_of_strings"},
                 "network": {"type": "string"},
                 "target": {"type": "string"},
-                "shm_size": {"type": ["integer", "string"]}
+                "shm_size": {"type": ["integer", "string"]},
+                "extra_hosts": {"$ref": "#/definitions/list_or_dict"}
               },
               "additionalProperties": false
             }