Presently the rowndcube app cannot be used behind a SSL proxy (or any reverse proxy, afaik).
Using ownCloud 5.0, only two minor changes are required:

* use OC_Request::serverHost() instead of $_SERVER['HTTP_HOST'] to determine rcHost in RoundcubeLogin.class.php
* check for the overwritewebroot configuration item in mail.php to include mailFrameScripts.js.