Dokku doesn't have an _official open source_ UI. There are a few unofficial OSS ones (Ledokku is the latest) that I'm aware of.

There is have a commercial offering in Dokku Pro (https://pro.dokku.com). It's paid (one-time lifetime license) but only so that I can at least partially cover my development time on it. The project is enough work on top of Dokku that I feel it is justified, especially as there is nothing stopping others from doing so, OSS or otherwise.