Dokku is an open-source, self-hosted platform as a service (PaaS), described as a Docker-powered mini-Heroku and an alternative to Heroku. You push an application to your own server with Git; Dokku builds it using Heroku buildpacks and runs it in isolated containers, so the lifecycle from build to scaling runs on a single host. It installs with a bootstrap script on a fresh Ubuntu or Debian machine, on amd64 or arm64.
Dokku is released under the MIT licence and developed in the open on GitHub. There is no hosted service: you run it on hardware or a virtual machine of your choice, so you decide where the applications and their data live, including at a European provider or in your own data centre. No vendor sits between you and your data, which fits organisations that want a simple PaaS without a foreign cloud dependency.