Update reboot-required file with staged image context

This change updates the /run/reboot-required file to include information
about the staged bootc image digest. This will provide users with
information about what bootc image will be effective after the system
has been rebooted.

Fixes #1574
Signed-off-by: Brendan Shephard <[email protected]>

Author: bshephar

crates/lib/src/deploy.rs

@@ -794,9 +794,13 @@ pub(crate) async fn stage(
 
     // Unconditionally create or update /run/reboot-required to signal a reboot is needed.
     // This is monitored by kured (Kubernetes Reboot Daemon).
+    let reboot_message = format!(
+        "Reboot required for bootc image digest: {}",
+        image.manifest_digest.as_ref()
+    );
     let run_dir = Dir::open_ambient_dir("/run", cap_std::ambient_authority())?;
     run_dir
-        .atomic_write("reboot-required", b"")
+        .atomic_write("reboot-required", reboot_message.as_bytes())
         .context("Creating /run/reboot-required")?;
 
     Ok(())