Earlier quoted context omitted.
Could you tell us exactly which part of Docker you think is "quite closed"? Docker (Moby) is licensed under Apache 2.0, and so is containerd, the default runtime.
The tight coupling to systemd which refuses to standardise any "internal" interfaces and insists on taking over the entirety of one's system. You can't run Docker if you're not running systemd, you can't run systemd if you're not running udevd and logd and so on, you can't run those if you're not running not just Linux but essentially only the "current" version of Linux, and if you ever implement a compatible replace…
I have historically had many problems with Docker and how the project is governed. I'm also no fan of systemd. But there's no point in literally spreading lies.