user credentials

[cgwalters]

We had a realtime chat on what specifically we should support configuring when generating disk images (as opposed to injecting into the container image). @mvo5 mentioned that the test suite right now relies on injecting a root ssh key. Yes, but note that it's also possible to inject a key in two other ways:

• As a derived container image
• Via systemd credentials (as is done in e.g. devshell: Support -S/--root-ssh-smbios coreos/coreos-assembler#3712 and https://gitlab.com/germag/osc/ and other places)

That said, note that containers/bootc@e477e4f also landed, so I think it would clearly make sense to expose that too, particularly after #18 is done.

There is still a much thornier question of the general problem of secrets, to which it's hard to have one opinionated answer (this touches on containers/bootc#22 etc.)

[cgwalters]

As a derived container image

Also I did #135 earlier related to this...I think it's definitely good for bib to test the generic base image, but arguably the first and most important test is (one or more) derived images, with the base image more as a special case.

[cgwalters]

Eh, this one isn't actionable; closing in favor of #143