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>

backend/Dockerfile.ffmpeg

@@ -5,6 +5,9 @@ WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci --ignore-scripts
 
+# Cache-bust: change CACHEBUST value in docker-compose.yml to force rebuild
+ARG CACHEBUST=1
+
 COPY . .
 RUN npm run build
 RUN npm prune --production