Docker + Nginx: Websocket returns 404 not found - node.js

Question

I'm struggling on implementing a websocket connection between a SSL Server and the client.
Architecture:
Proxy: Nginx
Host: Docker (Swarm)
Webserver: Node.js (express)
Client (Postman, later vue.js)
Nginx settings (app.conf):
server {
listen 443;
listen [::]:443;
client_max_body_size 100M;
server_name search.app search.app.host.ads;
location / {
proxy_pass http://search-service:3020; # docker container
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
}
}
search-server (server.js)
async function startServer() {
// variable definition
server = express();
// set http server
let httpServer = http.createServer(server);
// init loaders -> websocket is defined here
loaders(server, httpServer);
}
search-service (websocket.js) (how is the websocket created?)
let wsSearch = new websocket.Server({ server: httpServer, path: "/socket/websocketSearch" });
The websocket is working properly on localhost using this url
ws://localhost:3020/socket/websocketSearch
After deploying on production site, the url will be
wss://search.app.host.ads/socket/websocketSearch
Trying to connect to production websocket using Postman returns following error:
Error: Unexpected server response: 404
Handshake Details
Request URL: https://search.app.host.ads/socket/websocketSearch
Request Method: GET
Status Code: 404 Not Found
Request Headers
Sec-WebSocket-Version: 13
Sec-WebSocket-Key: gzRuxZ2QYTOladlXSenjmw==
Connection: Upgrade
Upgrade: websocket
Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits
Host: search.app.host.ads
Response Headers
Server: nginx/1.19.4
Date: Tue, 03 Aug 2021 13:07:45 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 161
Connection: keep-alive
X-Powered-By: Express
Access-Control-Allow-Origin: *
Content-Security-Policy: default-src 'none'
X-Content-Type-Options: "nosniff"
Which package do I use to implement websocket?
WS Version 7.5.3
I've followed several instructions like Nginx or related StackOverflow issues. However, I didn't manage to connect to my websocket.
Do you have any idea, where is my fault?
Thanks in advance.
If any further information is needed, I try to provide it.
Best regards