feat(gitlab-runner): Fix hetzner runner unable to use SSH

It seems that SSH requires an existing user in /etc/passwd

References:
https://unix.stackexchange.com/questions/524268/running-git-or-ssh-client-in-docker-as-user-no-user-exists-for-uid
https://superuser.com/questions/1704482/run-ssh-as-a-non-existent-virtual-user
https://unix.stackexchange.com/questions/766429/ssh-fails-with-no-user-exists-for-uid-x-on-ldap-user-on-ubuntu
https://github.com/openshift/jenkins/issues/168