Woodpecker is a simple CI engine with great extensibility.
OpenSource and free
Woodpecker is and always will be totally free. As Woodpecker's source code is open-source you can contribute to help evolving the project.
Based on docker containers
Woodpecker uses docker containers to execute pipeline steps. If you need more than a normal docker image, you can create plugins to extend the pipeline features. How do plugins work?
Woodpecker allows you to easily create multiple pipelines for your project. They can even depend on each other. Check out the docs