Merge pull request #6025 from vdemeester/init-in-3.7

Add `init` support in 3.7 schema

compose/config/config_schema_v3.7.json

@@ -154,6 +154,7 @@
         "healthcheck": {"$ref": "#/definitions/healthcheck"},
         "hostname": {"type": "string"},
         "image": {"type": "string"},
+	"init": {"type": "boolean"},
         "ipc": {"type": "string"},
         "isolation": {"type": "string"},
         "labels": {"$ref": "#/definitions/list_or_dict"},