fix: add cache-bust build arg to force Docker rebuild on deploy

Portainer stack updates were serving stale cached Docker layers.
Added CACHE_BUST ARG so each deploy can force a fresh build.
Set CACHE_BUST env var in Portainer to current timestamp to trigger.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -1,6 +1,9 @@
 services:
   botfights:
-    build: .
+    build:
+      context: .
+      args:
+        CACHE_BUST: ${CACHE_BUST:-0}
     container_name: botfights
     restart: unless-stopped
     ports: