gitlab-ci: Enable secret detection

This patch should provide automated secret detection, preventing any
secrets from sneaking out unnoticed. This is part of GitLabs AutoDevOps
feature set.

Reference:
https://docs.gitlab.com/ee/user/application_security/secret_detection/index.html
https://gitlab.com/gitlab-org/gitlab/-/blob/f006fea31097510743c47604f836915b088a6b5c/lib/gitlab/ci/templates/Security/Secret-Detection.gitlab-ci.yml