fix(docker): clear build cache on every build to stop unbounded disk growth

scripts/docker-update.sh

@@ -79,6 +79,9 @@ fi
 log "Node major gate OK (major=${float_major:-?})"
 
 # --- 4. Rebuild the image ---
+# Reset the BuildKit cache first so the build doesn't accumulate unbounded
+# layers on disk across daily rebuilds.
+docker builder prune -af --filter "until=1h" --keep-storage=0 2>>"$LOG_FILE" || true
 docker compose build >>"$LOG_FILE" 2>&1 || die "docker compose build failed" 2
 log "docker compose build OK"