Prow
This approach would continue to leverage Prow as the OKD build system.
Images would be built pre-submit (as CI on PRs) and post-submit (on merge, promoted to OKD ImageStream)
Pros
Direct feedback to devs (on status) through GitHub UI
Source of truth build metadata from Prow
Prow has ability to replace FROM directives in Dockerfiles
Continuous builds (ie. components are rebuilt immediately after they've been changed)