WebSocket connection between two docker containers - node.js

Question

Context
I've been working on GraphQL subscriptions for my team and have been trying to set up an API Gateway WebSocket API that listens to incoming requests and forwards the subscriptions to our backend services. The integration for the WebSocket API is a VPC Link that targets an ECS cluster which is running a server. One issue with VPC Link integrations for WebSocket APIs is that the WebSocket request is automatically downgraded to HTTP. Therefore, I've been exploring an alternative solution involving a proxy HTTP server which listens to the downgraded WebSocket request and then establishes a WebSocket request with the actual WebSocket server. The proxy HTTP server is defined with ExpressJS and the WebSocket server uses graphql-ws (similar to this example from Apollo https://www.apollographql.com/docs/apollo-server/data/subscriptions/#enabling-subscriptions, the full server code example listed in item #7).
Problem
My problem is that I am unable to successfully establish a WebSocket connection between the two servers when testing the docker containers locally. The current setup is two local docker containers (HTTP proxy and WebSocket server) that are running on the default "bridge" network. Once both servers are running, I use Postman to send a PUT request to the HTTP proxy server. Ideally, the HTTP proxy server should then open a WebSocket connection with the WebSocket server. However, this leads to a 4406 error: Subprotocol not acceptable. graphql-ws uses the graphql-transport-ws protocol (https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md) and I've tried to change the protocol defined in the WebSocket class constructor but 4406 error is happening regardless of whether or not I specify a protocol on the new WebSocket.
I've tested the WebSocket server by sending requests directly to it and it works as expected. This is why I believe the issue is the HTTP proxy server and not the WebSocket server. Here is a TypeScript code sample that demonstrates how I'm creating the WebSocket on the proxy server:
import express from 'express';
import WebSocket from 'ws';
const app = express();
app.put('/', (req, res) => {
const body = req.body ? req.body : {};
const socket = new WebSocket(wsEndpoint, {
headers: {
'Sec-WebSocket-Version': 13,
'Sec-WebSocket-Key': 'Ab2SMOa5hg0YUyC1OELtyQ==',
Connection: 'Upgrade',
Upgrade: 'websocket',
'Sec-WebSocket-Extensions':
'permessage-deflate; client_max_window_bits',
Host: 'localhost',
},
protocol: 'graphql-transport-ws',
});
socket.on('error', (err) => {
console.log(JSON.stringify(err));
});
socket.on('open', (s: any) => {
console.log('socket open');
});
socket.on('close', (code, reason) => {
console.log(`socket close: ${code}, reason: ${reason.toString()}`);
});
socket.on('error', (error) => {
console.log('socket error');
console.log(error);
});
socket.on('upgrade', (socket: any, request: any) => {
console.log('socket upgrade');
console.log(request);
});
res.status(200).json({
message: 'Connection established to WebSocket server',
});
});
Log snippet:
Proxy server listening on port 80 <-- app.listen() on port 80
socket upgrade <-- starts once I make the PUT request via Postman
socket open
socket close: 4406, reason: Subprotocol not acceptable
What I've Tried
So far I've tried to open the WebSocket both with a protocol in the WebSocket class constructor, and without a value. However, the 4406 error still shows up in both cases.
I've also tried to follow advice that was discussed in this question: How to create websocket connection between two Docker containers. When I create a user defined network, I still run into this exact same 4406 error.
I suspect that it might be that I'm missing something in the WebSocket constructor or some low-level knowledge about how graphql-ws works.