Remove -a args from docker ps command

The 'docker ps -a | ...' command in get_postgresql_id() list all
containers those matched the given app name even they are not running,
in case it has more than one result,  Dokku failed to retrieve the port
of currently running Postgresql container and cannot set a new
DATABASE_URL to the app after rebooting.
Add dokku postgresql:link to the end of pre-deploy

The new port of PG containre is not set into the DATABASE_URL after
rebooting, so the app container is fail to start. Adding a 'dokku
postgresql:link' to the end of pre-deploy file can fixs this.