Depends how your starting the VM. I’ve run Docker on Firecracker with a Raspberry PI 4 before but it needed some fixes.

One possibly is if your running directly from a Initramfs without a block device then docker needs DOCKER_RAMDISK set as a environment variable.

Otherwise it’s possible the minimal kernel your Firecracker config uses doesn’t support it out of the box. You can use a regular kernel but you need to make sure modules can be loaded from somewhere.