checker fixes

receiver/challenges/CDN.py

@@ -26,7 +26,7 @@ class CDN(Challenge):
     flag_location = 'flags/cdn.txt'     # host copy (written by your orchestrator)
     history_location = 'history/cdn.txt'
     container_flag_path = '/flag.txt'
-    container_name = 'chal_app'            # adjust if your container name differs
+    container_name = 'cdn_container'            # adjust if your container name differs
 
     # Canonical ExifTool markers (NO app fallbacks allowed)
     _exif_must_have = (