nginx with websockets for a local phoenix server

nginx.conf

map $http_upgrade $connection_upgrade {
        default upgrade;
        '' close;
    }

upstream useful-api {
    server 127.0.0.1:4000;
}

server {
    listen 80;
    server_name useful-api.dev;
    client_max_body_size 100M;

    try_files $uri/index.html $uri.html $uri @app;

    location @app {
      proxy_pass http://useful-api;

      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Host $http_host;
      proxy_http_version 1.1;
      proxy_redirect off;

      proxy_set_header Upgrade $http_upgrade;
      proxy_set_header Connection $connection_upgrade;
    }
}