Update release basejail datasets after updating a release

[gronke]

When a release that was already copied to the collection of base datasets (/iocage/base/<RELEASE>), the datasets representing the datasets need to get updated as well.

I wonder what the most efficient way to do that is. Here are some options:

Run freebsd-update a second time

+ does not need to write/touch all files
- HardenedBSD currently does not cache assets on the host (yet)
- It it another source of errors that require our action (e.g. roll back changes from the previous release update, so that the release and the base do not diverge)

Copy over the files

+ simple
- slow
- reads and writes back from disk
- errors like full disks may still require our action to handle rollbacks

Suggestions welcome

[gronke]

Solved by using rsync in 03c00b0