Allow container layers for firecracker-pilot

[schaefi]

Description

So far the firecracker-pilot can start the given VM and provision an optional include tarball. It would be nice to extend the provision step to allow also containers to be provisioned into the VM such that the same container based workload can easily be moved into a VM based workload. From a registration perspective this can look like

flake-ctl firecracker register --vm base --oci-layer container --app some-app --overlay-size 20GiB

An overlay is required for this provision step to work in the same way as for the tar include

Acceptance Criteria

• OCI containers can be layered with firecracker VM instances