The systemd-nspawn system (which is how running more more typical containers is normally implemented) is a little unusual because it it geared towards running o

The systemd-nspawn system (which is how running more more typical containers is normally implemented) is a little unusual because it it geared towards running os containers, including running their whole init system (via --boot or the template unit file). There is a reason why machinectl calls the input machine images, and both accept raw disk images. This is also reflected in the systemd-nspawn requirement of having an os-release file in the container. Of course, it is entirely capable of running application containers (hence features like --as-pid2), but some of the defaults like registering with systemd-machined don't make as much sense in those scenarios. Docker is nominally geared towards application containers, although it mostly works with OS containers. Indeed it seems like container images that contain a full OS (minus kernel) are probably more common than not.