Dokku & Nginx - Connection failed while connecting to upstream (code 111)

Question

I use Dokku to host my React app and Nginx for proxy.

I get a 502 Bad Gateway error, when I try to access on the homepage...

dokku nginx:error-logs app give me

2020/07/16 06:15:45 [error] 16459#16459: *308 connect() failed (111: Connection refused) while connecting to upstream, client: 92.184.xxx.xxx, server: api.example.com, request: "OPTIONS / HTTP/2.0", upstream: "http://172.17.0.12:5000/", host: "api.example.com", referrer: "https://example.com/"

I reset the application network, its configuration, and its proxy settings.

Dokku Nginx configuration:

server {
  listen      [::]:80;
  listen      0.0.0.0:80;
  server_name api.example.com; 
  access_log  /var/log/nginx/app-access.log;
  error_log   /var/log/nginx/app-error.log;

  return 301 https://$host:443$request_uri;

}

server {
  listen      [::]:443 ssl http2;
  listen      0.0.0.0:443 ssl http2;
  
  server_name api.example.com; 
  access_log  /var/log/nginx/app-access.log;
  error_log   /var/log/nginx/app-error.log;

  ssl_certificate           /home/dokku/app/tls/server.crt;
  ssl_certificate_key       /home/dokku/app/tls/server.key;
  ssl_protocols             TLSv1.2 TLSv1.3;
  ssl_prefer_server_ciphers off;

  keepalive_timeout   70;
  

  location    / {

    gzip on;
    gzip_min_length  1100;
    gzip_buffers  4 32k;
    gzip_types    text/css text/javascript text/xml text/plain text/x-component application/javascript application/x-javascript application/json application/xml  application/rss+xml font/truetype application/x-font-ttf font/opentype application/vnd.ms-fontobject image/svg+xml;
    gzip_vary on;
    gzip_comp_level  6;

    proxy_pass  http://app-5000;
    http2_push_preload on; 
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection $http_connection;
    proxy_set_header Host $http_host;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-Forwarded-For $remote_addr;
    proxy_set_header X-Forwarded-Port $server_port;
    proxy_set_header X-Request-Start $msec;
  }
  include /home/dokku/app/nginx.conf.d/*.conf;

  error_page 400 401 402 403 405 406 407 408 409 410 411 412 413 414 415 416 417 418 420 422 423 424 426 428 429 431 444 449 450 451 /400-error.html;
  location /400-error.html {
    root /var/lib/dokku/data/nginx-vhosts/dokku-errors;
    internal;
  }

  error_page 404 /404-error.html;
  location /404-error.html {
    root /var/lib/dokku/data/nginx-vhosts/dokku-errors;
    internal;
  }

  error_page 500 501 503 504 505 506 507 508 509 510 511 /500-error.html;
  location /500-error.html {
    root /var/lib/dokku/data/nginx-vhosts/dokku-errors;
    internal;
  }

  error_page 502 /502-error.html;
  location /502-error.html {
    root /var/lib/dokku/data/nginx-vhosts/dokku-errors;
    internal;
  }
}

upstream app-5000 {

  server 172.17.0.12:5000;
}

My application listens on port 5000 (used by Dokku), and for the host, I tried 127.0.0.1, then 0.0.0.0, but it still does not work.

I don't know where to look, or even what to do.

I have too a Procfile:

web: npm run start
api: npm run start-api