Update reboot-required file with staged image context

This change updates the /run/reboot-required file to include information
about the staged bootc image. 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 <bshephar@bne-home.net>

Author: bshephar

crates/lib/src/deploy.rs

@@ -794,9 +794,10 @@ 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: {}", &spec.image.image);
     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(())