Tekton Pipeline is a bit different than other candidates we have on the list but nonetheless a compelling option. It's essentially a set of k8s custom resource definitions (CRDs) that can constitute a basic CI/CD system. Whether this alone, or with minimal extension, could make up a sufficient system is not clear. Other systems such as Jenkins X seem to be evaluating it as an underlying framework.
A good background on the project, as a subproject of the Knative Community Project, can be found here.