Add flag to stop containers before updating

[markdumay]

The script invokes synoservicectl --stop pkgctl-Docker to stop the Docker daemon. Certain containers prevent a successful shutdown, possibly linked to Docker's Live Restore feature.

Usually, manually stopping any running containers prior to running syno_docker_update.sh does the trick. A new flag (e.g. --force-stop) for all commands except download is to automate this step. Possibly, the stopped containers can also be automatically restarted once the script has completed.