image is not eligible for automated image updates

Yo, loving Fly so far, just a quick question regarding fly image update: when I run this I get “image is not eligible for automated image updates” but there’s no information about what makes an image “eligible” in the docs or in the error message.

I thought it might be that updates only work for images tagged with :latest so I re-tagged my image but this was not the case.

For now I’ll just update my terraform config with each explicit image version but it would be neat to run one command to bump the version without needing to edit the tf config.