It doesn't have to be a container. We have experimented with using VMs as the CNAB runtime as seen with the azure-vm driver in our reference implementation, duffle: https://github.com/deislabs/duffle/tree/master/drivers/azure...

Most of the examples are primarily container-based and the specification reflects that. We will definitely have to do a better job fleshing out the design with alternative invocation image types than OCI/docker. The azure-vm driver is one such (experimental) example.

Hope this helps!