Since version 8.0 GitLab CI has been merged into GitLab CE. You no longer need to setup a separate CI instance.
Refer to https://github.com/sameersbn/docker-gitlab/blob/master/CI_MIGRATION.md for the migration instructions.
PSA: BACKUP YOUR DATA BEFORE MIGRATING
If you are interested in older versions of GitLab CI, please refer to the README of the tagged versions.