Unable to setup certbot properly with nginx + rails

According to the article https://www.digitalocean.com/community/tutorials/how-to-secure-nginx-with-let-s-encrypt-on-ubuntu-16-04

I’m running command to test it:

    $ sudo certbot renew --dry-run  

And get this:

      WARNING:certbot.renewal:Attempting to renew cert from /etc/letsencrypt/renewal/my_website.com.conf produced an unexpected error: At least one of the (possibly) required ports is already taken.. Skipping.
      ** DRY RUN: simulating 'certbot renew' close to cert expiry
      **          (The test certificates below have not been saved.)

      All renewal attempts failed. The following certs could not be renewed:
        /etc/letsencrypt/live/my_website.com/fullchain.pem (failure)
      ** DRY RUN: simulating 'certbot renew' close to cert expiry
      **          (The test certificates above have not been saved.)

It’s because nginx is running. But: how is it supposted to work? Stopping nginx and my website twice a day to check and possibly renew a certificate won’t be a wise way to go

How to fix it?

I’m using nginx + rails.

In cron I have this job:

    $ sudo certbot renew