This could be done using a system cron job which runs

  clace app update-metadata conf --promote container.idle_shutdown_secs=180 /myapp

at every 6pm and then runs

  clace app update-metadata conf --promote container.idle_shutdown_secs=0 /myapp

at 8am. idle_shutdown_secs being zero disables the shutdown. Using all as the last arg will update for all apps.