name one (1) CI system, open or closed, which shares enough with another CI system, open or closed, that there is no pain when changing from one to another.
And running your jobs in Docker is what I recommend that people at my work do, and I admin several GitHub Enterprise Server instances and GitHub Enterprise Cloud as well.
This is the one thing that Drone got very right - every Drone job runs in a container. It is built in to the drone tooling to be able to run those jobs locally on your development machine as well, and requiring containers is why that works.
If you run your CI and/or CD steps from within a container, you can run that container anywhere, and writing a small script to read your CI/CD yaml (or whatever you use), and wrap your favorite container command line tool into a working local CI/CD system should be pretty trivial.
Using containers also makes moving to another CI/CD system which can use containers as trivial as it can currently be.
they are all at least semi proprietary.