Gitlab CI currently allows building flatpak on experimental basis. Overall recipe for the flatpak builds is subject to change. Currently `fiesta-flatpak` runner is locked to selected projects, if you want to enable the flatpak builds ping @bshah.
Add `binary-flatpak.yml` template in the your `.gitlab-ci.yml`
Currently template downloads the flatpak manifest from the `flatpak-kde-applications` repository. This is subject to change, and in future we might ship flatpak manifest in repo directly.