json middleware is part of Express v4.16.0

src/dockerproxy.js

@@ -118,7 +118,7 @@ function process(req, res, next) {
 async function start() {
     assert(gHttpServer === null, 'Already started');
 
-    const json = middleware.json({ strict: true });
+    const json = express.json({ strict: true });
 
     // we protect container create as the app/admin can otherwise mount random paths (like the ghost file)
     // protected other paths is done by preventing install/exec access of apps using docker addon