fix(docker): actually use CACHE_BUST ARG to bust Docker layer cache

The ARG was defined but never used in a RUN/ENV command, so Docker
ignored value changes and kept using cached COPY layers from the
very first build. Adding 'RUN echo' forces cache invalidation.

Dockerfile

@@ -4,7 +4,9 @@ WORKDIR /app
 
 # --- Install + Build in one stage ---
 FROM base AS builder
+# CACHE_BUST: change this value to force a full rebuild (busts Docker layer cache)
 ARG CACHE_BUST=8
+RUN echo "Cache bust: ${CACHE_BUST}"
 COPY . .
 RUN pnpm install --no-frozen-lockfile
 ENV NEXT_TELEMETRY_DISABLED=1