systemd-nspawn
systemd comes with a container solution. It started as an experiment, and then Lennart Poettering considered it ready for production. It is, in fact, the base for another solution, Rkt. At the time of writing this book, Rkt development has stopped. However, you can still access the Rkt GitHub repository (https://github.com/rkt/rkt).
systemd-nspawn is not very well known, but it is a powerful solution that is available on every modern Linux system. It is built on top of the kernel namespaces and systemd for management. It's a sort of chroot on steroids.
If you want to learn more about the underlying technologies of containers, systemd-nspawn is a good start. Here, every component is visible and can be configured manually if you want. The downside of systemd-nspawn is that you have to do everything on your own, from creating the image, to orchestration, to high availability: it's all possible, but you have to build it.