Backing up the GitLab secrets file is required to avoid complicated steps when recovering GitLab from backup. The secrets file is stored at /etc/gitlab/gitlab-secrets.json inside the container, or $GITLAB_HOME/config/gitlab-secrets.json on the container host.
See also https://adaptive.u-aizu.ac.jp/gitlab/yshimmyo/configuration-of-adaptive-u-aizu-ac-jp