--download-only should store/load kicbase on local filesystem

[tstromberg]

To ensure minikube can startup in an air-gapped environment.

Related issue: #10149

[afbjorklund]

Currently it is only implemented for daemon, there is nothing in the cache - or for the other engines, such as podman

        if cc.Driver != "docker" {
                // TODO: driver == "podman"
                klog.Info("Driver isn't docker, skipping base image download")
                return
        }

The logic today is quite weird, if we fail saving the image to the daemon - then we try saving it to the cache ?!?

                        err := image.WriteImageToDaemon(img)
                        if err == nil {
                                klog.Infof("successfully downloaded %s", img)
                                finalImg = img
                                return nil
                        }
                        if downloadOnly {
                                if err := image.SaveToDir([]string{img}, constants.ImageCacheDir); err == nil {
                                        klog.Infof("successfully saved %s as a tarball", img)
                                        finalImg = img
                                        return nil
                                }
                        }

[afbjorklund]

Another ironic thing is that in order to save it to the docker daemon, go-registry first downloads a copy to a tempfile...

EDIT: Actually it was a pipe and not a tempfile, I remembered wrong. So it's like: crane pull | docker load

[afbjorklund]

I think we discussed this feature in issue #7766, but settled for the daemon-only (docker-only) solution.

I suppose that technically /var/lib/docker is a local filesystem ? But I assume this issue means cache.

[afbjorklund]

@tstromberg : should this be stored under "images", along with kubernetes and addons

or should it be stored in a top-level directory, similar to "iso" (maybe something like "kic")