Use variable instead of hardcoded value and document the reasoning

roles/gitlab/tasks/main.yml

@@ -28,13 +28,15 @@
       # https://gitlab.com/gitlab-org/gitlab/issues/14414
       #
       # In addition, see https://docs.gitlab.com/ee/administration/pages/ for the GitLab Pages trickery done below.
+      # Basically, we only allow specific GitLab Pages with custom domains to work. We don't want to enable everyone
+      # to be able to have a GitLab Page on purpose (for security and legal safety reasons).
       GITLAB_OMNIBUS_CONFIG: |
         external_url 'https://{{ gitlab_domain }}'
         nginx['client_max_body_size'] = '2g'
         nginx['listen_addresses'] = {{ gitlab_primary_addresses }}
         registry_nginx['listen_addresses'] = ['*', '[::]']
         gitlab_pages['inplace_chroot'] = true
-        pages_external_url "http://gitlab.archlinux.org"
+        pages_external_url "http://{{ gitlab_domain }}"
         pages_nginx['enable'] = false
         gitlab_pages['external_http'] = {{ gitlab_pages_http_addresses }}
         gitlab_pages['external_https'] = {{ gitlab_pages_https_addresses }}