A 'container image' is a snapshot of a container. You can host your container images with GitLab.
To start using container images hosted on GitLab you first need to login:
docker login registry.gitlab.com
Then you are free to create and upload a container image with build and push commands:
docker build -t registry.gitlab.com/gitlab-org/omnibus-gitlab/image .
docker push registry.gitlab.com/gitlab-org/omnibus-gitlab/image