Let's Encrypt doesn't work on fresh install

Summary

Let's Encrypt gives me an error when I set the external url and set letsencrypt to enabled. Error can be seen here: https://forum.gitlab.com/t/lets-encrypt-error/32357 I have Gitlab installed within a Docker container which was provided in the official installation guide. I'm using Ubuntu 18.04

Steps to reproduce

(How one can reproduce the issue - this is very important)

What is the current bug behavior?

SSL certificate cannot be created

What is the expected correct behavior?

SSL certificate to be deployed correctly

Relevant logs and/or screenshots

https://forum.gitlab.com/t/lets-encrypt-error/32357

Results of GitLab environment info

I'm using the Docker image, idk how to get this