download without applying (finalization locking)

[jlebon]

(edited by @cgwalters )

We should support e.g. bootc upgrade --lock-finalization which would do the same thing as https://github.com/coreos/rpm-ostree/pull/4939

This would allow downloading an update without actually applying it.

Original issue:

This was mentioned in https://github.com/bootc-dev/bootc/issues/165 as a desirable feature, but AFAICT there isn't a card tracking it specifically.

Obviously, this isn't hard to do I think with the current OSTree backend. Though the expectation is that we'll want to retain this semantic when we eventually move to a pure composefs-based backend.

[cgwalters]

This issue also relates to https://github.com/bootc-dev/bootc/issues/1375

[urld]

@jlebon is there any chance to also allow this flag for bootc-switch? Working on a project where we are currently using bootc-switch rather than bootc-upgrade as we need to be able switch to exact versions, rather than relying on the latest available. At the moment we download the images outside of bootc and then use bootc switch --transport oci. Having the --download-only flag would make this more streamlined.