fix: use Alpine-compatible flags for addgroup/adduser in frontend Dockerfile

node:20-alpine uses BusyBox which does not support GNU-style
--system/--ingroup flags. Switch to -S/-G equivalents.

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

frontend/Dockerfile

@@ -39,7 +39,7 @@ COPY --from=builder /app/node_modules ./node_modules
 COPY --from=builder /app/next.config.ts ./next.config.ts
 
 # Run as non-root user
-RUN addgroup --system appgroup && adduser --system --ingroup appgroup appuser \
+RUN addgroup -S appgroup && adduser -S -G appgroup appuser \
   && chown -R appuser:appgroup /app
 USER appuser