wishlist: make timeout on non-empty workdir configurable

while creating and re-creating my roundcube container (in the wake of #380 ) i got increasingly annoyed by the sleep time in https://github.com/roundcube/roundcubemail-docker/blob/a0db4b1bb531bbfbab0f3660744a9c6bb7c6bbfe/templates/docker-entrypoint.sh#L40-L43 I *know* that my `/var/www/html` is not empty, because i bind-mount my personal-skin to `/var/www/html/skins/umlaeute`, so there is no reason to make me wait whenever I create a new container. I also understand that you might want to keep this sleep as a safeguard for misconfigurations. so how about allowing the user to override the timeout? like so: ```bash ( set -x; ls -A; sleep "${ROUNDCUBE_EMPTYWORKDIR_TIMEOUT:-10}" ) ``` which allows me have `ROUNDCUBE_EMPTYWORKDIR_TIMEOUT=0` in my `.env` file to just skip the sleep (obviously this could be improved, to not emit the "*Press ^C to abort*" if the timeout is 0; and whatnot)