A nice addition to the “Blueprints” section of the official docs was merged recently, and, among other things, it includes the first explicit guarantee about how long images really persist on registry.fly.io:
https://fly.io/docs/blueprints/using-base-images-for-faster-deployments/
[…] as long as an image is associated with a Machine–even if the Machine is stopped–it won’t be deleted.
This was a topic of some debate in the forum earlier, in the context of workarounds for the lack of access to custom private registries, so it’s a relief that there’s been a final word on the topic now.
This is part of a broader trend, in my view, of more and more things getting finally nailed down in the docs…
The overall technique of uploading an image once and then reusing it in multiple apps is rather elegant on its own, allowing for some pretty fancy choreography, 
…