This allows the `linuxkit/kubernetes` "image-cache" packages to use a standard
`linuxkit pkg build` based flow rather than requiring surrounding scaffolding.
Fixes#2766. Compared with the original (actually, the second) proposal made in
issue #2766, the field is `docker-images` rather than `images` to allow for
future inclusion of e.g. `containerd-images`.
Signed-off-by: Ian Campbell <ijc@docker.com>