I run a bunch of docker containers on Synology and on a few servers, and use watchtower to update their images automatically.

First set the timezone to Taipei:

sudo timedatectl set-timezone Asia/Taipei

Add the cleanup and schedule flags:

docker run -d --restart=always \
--name watchtower \
-v /var/run/docker.sock:/var/run/docker.sock \
-v /etc/localtime:/etc/localtime:ro \
containrrr/watchtower \
--cleanup \
--schedule "0 0 4 * * *"

More flags can be found here: https://containrrr.dev/watchtower/arguments/