Add CACHEBUST build arg to force Docker image rebuilds

Docker's build cache was preventing Portainer from picking up
code changes. Adding a CACHEBUST ARG before COPY invalidates
all subsequent layers when the value changes.

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

docker-compose.yml

@@ -20,6 +20,7 @@ services:
       context: .
       dockerfile: Dockerfile
       args:
+        CACHEBUST: "2"
         VITE_USE_MOCK_DATA: "false"
         VITE_CONTENT_ORIGIN: ${FRONTEND_URL}
         VITE_INDEEHUB_API_URL: /api
@@ -45,6 +46,8 @@ services:
     build:
       context: ./backend
       dockerfile: Dockerfile
+      args:
+        CACHEBUST: "2"
     restart: unless-stopped
     environment:
       # ── Core ─────────────────────────────────────────────
@@ -175,6 +178,8 @@ services:
     build:
       context: ./backend
       dockerfile: Dockerfile.ffmpeg
+      args:
+        CACHEBUST: "2"
     restart: unless-stopped
     environment:
       ENVIRONMENT: production