auto-update: add --dry-run

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>

1 files changed, 6 insertions, 2 deletions

diff --git a/pkg/domain/entities/auto-update.go b/pkg/domain/entities/auto-update.go
index d74462b86..eed617bf8 100644
--- a/pkg/domain/entities/auto-update.go
+++ b/pkg/domain/entities/auto-update.go
@@ -4,6 +4,10 @@ package entities
 type AutoUpdateOptions struct {
 	// Authfile to use when contacting registries.
 	Authfile string
+	// Only check for but do not perform any update.  If an update is
+	// pending, it will be indicated in the Updated field of
+	// AutoUpdateReport.
+	DryRun bool
 }
 
 // AutoUpdateReport contains the results from running auto-update.
@@ -18,7 +22,7 @@ type AutoUpdateReport struct {
 	Policy string
 	// SystemdUnit running a container configured for auto updates.
 	SystemdUnit string
-	// Indicates whether the image was updated and the container (and
-	// systemd unit) restarted.
+	// Indicates the update status: true, false, failed, pending (see
+	// DryRun).
 	Updated string
 }