deb: use overlayfs instead of copy

src/deb/ephemeral.rs

@@ -337,6 +337,16 @@ impl EphemeralContextGuard {
 impl Drop for EphemeralContextGuard {
     fn drop(&mut self) {
         log::debug!("Cleaning up ephemeral context ({:?})...", self.chroot_path);
+
+        // Clean up any overlay mounts before resetting the context.
+        // This must happen while the ephemeral context is still current so its
+        // driver is accessible. The actual unmount commands run via the parent
+        // (base) context, so they work regardless.
+        let ephemeral_ctx = context::current();
+        if let Err(e) = ephemeral_ctx.cleanup() {
+            log::warn!("Failed to clean up overlay mounts: {}", e);
+        }
+
         // Reset to normal context
         if let Err(e) = context::manager().set_current(&self.previous_context) {
             log::error!("Failed to restore context {}: {}", self.previous_context, e);