fix: add CACHEBUST arg to force Docker cache invalidation

Dockerfile

@@ -18,6 +18,8 @@ RUN flutter config --enable-web
 
 # Copy dependency files first for better caching
 # Note: pubspec.lock will be generated by flutter pub get if missing
+# Cache buster - change this value to force rebuild
+ARG CACHEBUST=2024-12-08-v1
 COPY pubspec.yaml ./
 
 # Get dependencies (generates pubspec.lock automatically)