what am I doing wrong in nginx / docker to get connect() failed (111: Connection refused) while connecting to upstream ? (SSL)

Question

I am trying to solve why I am getting on docker / nginx container error 502 with connect() failed (111: Connection refused) when I setup properly the ssl , but I got this issue . I dont see clerly where it comes from . how can I fix this?

upstream web_server {
    server web:8000;
}

server {

    listen 80;
    server_name demo.io www.demo.io;

    access_log /var/log/nginx/demo.io.log;
    error_log /var/log/nginx/demo.io.log;

    location / {
        rewrite ^ https://$host$request_uri? permanent;
        proxy_pass http://web_server;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $host;
        proxy_redirect off;
    }

    location /static/ {
        alias /home/app/web/static/;
    }
}
#https://demo.io
server {
    listen 443 ssl http2;
    listen [::]:443 ssl http2;
    server_name demo.io;

    server_tokens off;

    ssl_certificate /etc/letsencrypt/live/demo.io/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/demo.io/privkey.pem;

    ssl_buffer_size 8k;

    ssl_dhparam /etc/ssl/certs/ssl-dhparams.pem;

    ssl_protocols TLSv1.2 TLSv1.1 TLSv1;
    ssl_prefer_server_ciphers on;

    ssl_ciphers ECDH+AESGCM:ECDH+AES256:ECDH+AES128:DH+3DES:!ADH:!AECDH:!MD5;

    ssl_ecdh_curve secp384r1;
    ssl_session_tickets off;

    # OCSP stapling
    ssl_stapling on;
    ssl_stapling_verify on;
    resolver 8.8.8.8;

    location / {
        proxy_pass https://web;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $host;
        proxy_redirect off;
    }
}