extract images from the Installer and put in the `additionalImages` array

[mrsimonemms]

As the Installer manages the all the container images internally, these are not exposed to KOTS for airgapping purposes. To that end, we need to export the images from the Installer and put them in the additionalImages array in the kots-app.yaml file.

There's already a mirror list command which we should be able to use.

See KOTS docs