Add a --dry-run flag to `podman auto-update` which will look for new
images but won't perform any pull or restart any service or container.
The "UPDATED" column will now indicate the availability of a newer image
via "pending".
```
$ podman auto-update --dry-run
UNIT CONTAINER IMAGE POLICY UPDATED
container-test.service 08fd34e533fd (test) localhost:5000/busybox registry false
```
Fixes: #9949
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>
59f31d86ac