We ship Flatcar by default with Docker / Containerd, which suits most, but not all, use cases:

• some require older / newer Docker versions than what we ship
• some do not use Docker and work with containerd directly
• some use entirely different container runtimes like CRI-O or podman

Using systemd-sysext, we would be able to support a custom container stack at provisioning time, e.g.

• picking from multiple Docker versions, or don't use Docker at all but work with containerd directly
• use a different container runtime, e.g. crun instead of runc
• use an entirely different stack, like podman