Add bootc usr-overlay --read-only #2034
Description
Similar to ostree admin unlock --transient, bootc usr-overlay should have a --read-only/--readonly/--transient flag to create a read-only overlay instead of a writable overlay.
I would personally prefer --read-only over --transient. To me, "transient" sounds more like "disappears upon reboot" than "read-only". But I'm sure I'm missing some context from OSTree history.
We'd like to use this for dnf --transient, both dnf4 and dnf5. Currently, DNF 4 calls ostree admin unlock --transient which does not work with the composefs backend. dnf5 --transient is not yet implemented: rpm-software-management/dnf5#2358.
I'd be happy to work on this myself.