Have used Buildkite at a previous workplace. Really liked the UI. This is coming from someone who has used Jenkins, Travis, Codeship, and GitLab CI.

Their front-end is open source: https://github.com/buildkite/frontend

However, if you add your own CI runners, you'd have to clean them up periodically. This was the biggest issue for us, because even after cleaning them up from all docker images once a week, builds would get stuck.