Cannot connect to localhost, at any listening port, inside gitlab-ci

Summary

A pipeline runner cannot connect to localhost, at any port. Connection gets refused.

Steps to reproduce

1. Create an account (if you don't have one already) in gitlab.com
2. Create a new repo and put a .gitlab-ci.yml inside with:

test_backend_job:
  image: debian:latest
  script:
    - apt-get update && apt-get --assume-yes install netcat
    - nohup nc -l localhost 5000 &
    - sleep 5
    - nc -v localhost 5000

3. Commit, Push and let the pipeline run

This script uses netcat to listen to port 5000, and 5 seconds later tries to connect to it.

What is the current bug behavior?

(...)
$ nohup nc -l localhost 5000 &
$ sleep 5
$ nc -v localhost 5000
localhost [127.0.0.1] 5000 (?) : Connection refused
ERROR: Job failed: exit code 1

What is the expected correct behavior?

(...)
$ nohup nc -l localhost 5000 &
$ sleep 5
$ nc -v localhost 5000
Connection to localhost 5000 port [tcp/*] succeeded!
(...)