Because the containers provided by GitLab use the official omnibus package, all configuration actions are centered around the gitlab.rb file.

The software inside the container is provisioned using the omnibus GitLab install, so that means /etc/gitlab/gitlab.rb is used inside the container. You can edit the file by entering the container with a shell:

sudo docker exec -it gitlab /bin/bash

Another way is to directly edit the gitlab.rb file in a Docker command:

sudo docker exec -it gitlab vi /etc/gitlab/gitlab.rb

You will have to set external_url to something valid in the gitlab.rb file as well to make repository links in GitLab work correctly. When you are there, you can check other settings as well, such as enabling HTTPS, and very importantly, an SMTP server to use for mail. The Docker image does not have an SMTP server included.

When you are finished making the changes you want, you will have to restart the container to reconfigure GitLab (it does so every time at restart):

sudo docker restart gitlab