server {
    listen 443 ssl; # client_wss_port
    server_name www.example.org;

    ssl on;
    ssl_certificate /etc/letsencrypt/live/example.org/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.org/privkey.pem;


    location /chat/ {
        add_header locationischat 1; # this is a dummy header for debugging
        proxy_pass http://localhost:3000/chat/;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
    }
}

nginx -v
nginx version: nginx/1.10.3 (Ubuntu)

ufw status
Status: active

To                         Action      From
--                         ------      ----
OpenSSH                    ALLOW       Anywhere
80                         ALLOW       Anywhere
443                        ALLOW       Anywhere
8888                       DENY        Anywhere
3000                       DENY        Anywhere
OpenSSH (v6)               ALLOW       Anywhere (v6)
80 (v6)                    ALLOW       Anywhere (v6)
443 (v6)                   ALLOW       Anywhere (v6)
8888 (v6)                  DENY        Anywhere (v6)
3000 (v6)                  DENY        Anywhere (v6)

Request URL: wss://www.example.org/chat
Request Method: GET
Status Code: 301 Moved Permanently
Connection: keep-alive
Content-Length: 194
Content-Type: text/html
Date: Wed, 23 May 2018 10:42:35 GMT
Location: https://www.example.org/chat/
locationischat: 1
Server: nginx/1.10.3 (Ubuntu)
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Cache-Control: no-cache
Connection: Upgrade
Host: www.example.org
Origin: chrome-extension://omalebghpgejjiaoknljcfmglgbpocdp
Pragma: no-cache
Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits
Sec-WebSocket-Key: hFvk0oEAzI5FLVd4W2fgoA==
Sec-WebSocket-Version: 13
Upgrade: websocket
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.139 Safari/537.36

49.195.190.75 - - [23/May/2018:22:39:16 +0000] "GET /chat HTTP/1.1" 301 194 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.139 Safari/537.36"