prepull images should allow to prepull additional images

[jorgemoralespou]

If the workshop uses additional images (e.g. an s2i builder image) the workshop should be able to prepull it along with the rest of the workshop.

These should be a list of additional images to prepull.

[GrahamDumpleton]

I was going to add that, but what I wasn't sure of was whether it would work as desired in OCP 4, since you would be prepulling the images into the spawner project, where as the image is actually needed in the project created for the session. In OCP 3, images could effectively be used across users/projects because of how image caching works. I am not sure the same reuse occurs in OCP 4 because of how podman works. So deferred until can work it out.