How to configure NGINX to avoid 502 Bad Gateway when communicates with a Docker container?

Hi, my case is that after having set up a new Droplet with an Ubuntu 18.04 server and have followed the DO tutorials to:

1. Install Nginx
2. Secure Nginx with Let’s Encrypt
3. Install Jenkins
4. Configure Jenkins with an SSL behind an Nginx reverse proxy
5. Set up continuous integration pipeline with Jenkins (this doc says for Ubuntu 16.04)
6. Install docker

So the goal is to manage the deployment of a React application to my testdomain.com later, by now, I just want to create the dist folder generated, after the ‘npm build’, within the /var/lib/jenkins/workspace/ , just that. For the purpose, I will run it in a Jenkins pipeline with docker.

So by now, I’m able to access my jenkins.testdomain.com alright, trigger the pipeline to start the process after pushing to my repo, and start to run the stages; but it’s here when start to fail nginx, when the pipeline reaches the Deliver phase (let’s read ‘npm build’ phase). It’s at this point reading the Jenkins console output I see when it gets stuck in that phase to eventually show a 502 Bad Gateway. After this, I will need to make a systemctl restart Jenkins to have access again. After restarting the pipeline resume the work and seems to get the job done :/

In the /var/log/nginx/error.log for nginx I can read:

*1 connect() failed (111: Connection refused) while connecting to upstream, client: 85.146.85.194, server: jenkins.testdomain.com, request: “GET /job/Basic%20NodeJS-React%20app/8/console HTTP/1.1”, upstream: “https://127.0.0.1:8080/job/Basic NodeJS-React app/8/console”, host: “jenkins.testdomain.com”, referrer: “https://jenkins.testdomain.com/job/Basic NodeJS-React app/8/”

*1 SSL_do_handshake() failed (SSL: error:1408F10B:SSL routines:ssl3_get_record:wrong version number) while SSL handshaking to upstream, client: 85.146.85.194, server: jenkins.testdomain.com, request: “GET /favicon.ico HTTP/1.1”, upstream: “https://127.0.0.1:8080/favicon.ico”, host: “jenkins.testdomain.com”, referrer: “https://jenkins.testdomain.com/job/Basic NodeJS-React app/8/console” …

In the Jenkinsfile of my node-js-react app, the agent looks like this (pass tests from nginx -t)

pipeline {
    agent {
        docker {
            image 'node:6-alpine' 
            args '-p 3000:80' 
        }
    }
    environment {
            CI = 'true'
    }
    stages {
            // Build, Test, and Deliver stages
    }
}

And my jenkins.testdomain.com configuration (/etc/nginx/sites-available/jenkins.testdomain.com) is like this:

server {
    listen 80;

    root /var/www/jenkins.testdomain.com/html;
    server_name jenkins.testdomain.com www.jenkins.testdomain.com;

    location / {
      proxy_set_header    Host $host;
      proxy_set_header    X-Real-IP $remote_addr;
      proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;

      # Fix the "It appears that your reverse proxy set up is broken" error.
      proxy_pass          http://localhost:8080;

      # High timeout for testing
      proxy_connect_timeout 1200s;
      proxy_send_timeout 1200s;
      proxy_read_timeout 1200s;

      proxy_redirect      http://localhost:8080 https://jenkins.testdomain.com;

      # Required for new HTTP-based CLI
      proxy_http_version 1.1;
      proxy_request_buffering off;
  
      # Required for HTTP-based CLI to work over SSL
      proxy_buffering off;
    }

    # Certbot auto-generated lines...
}

Please, any help on this it would really helpful, after 2-3 days I’ve tried several things to avoid that error, but it gets back at some point.

Thanks in advance!