drpc: allow per-host DSHACKLE_IMAGE override for canary fork rollout

Default remains drpcorg/dshackle:0.79.10; canary hosts can set
DSHACKLE_IMAGE via host_vars env_overrides without editing compose.

Co-authored-by: Cursor <cursoragent@cursor.com>

drpc-free.yml

@@ -1,6 +1,6 @@
 services:
   dshackle-free:
-    image: drpcorg/dshackle:0.79.10
+    image: ${DSHACKLE_IMAGE:-drpcorg/dshackle:0.79.10}
     restart: unless-stopped
     volumes:
       - ./free_configs:/etc/dshackle

drpc.yml

@@ -23,7 +23,7 @@ services:
       sleep 10; done'
 
   dshackle:
-    image: drpcorg/dshackle:0.79.10
+    image: ${DSHACKLE_IMAGE:-drpcorg/dshackle:0.79.10}
     restart: unless-stopped
     volumes:
       - ./main_configs:/etc/dshackle