ostree-admin-unlock man page
ostree-admin-unlock — Prepare the current deployment for hotfix or development
ostree admin unlock [Options...]
Remove the read-only bind mount on /usr and replace it with a writable overlay filesystem. This default invocation of "unlock" is intended for development/testing purposes. All changes in the overlay are lost on reboot. However, this command also supports "hotfixes", see below.
If this option is provided, the current deployment will be cloned as a rollback target. This option is intended for things like emergency security updates to userspace components such as sshd. The semantics here differ from the default "development" unlock mode in that reboots will retain any changes (which is what you likely want for security hotfixes).