Add possibilty to add public assets to public_html

The current suggestion is to mount the whole `/var/www/html` folder. This makes it impossible to build a derived image, which already contains things within this path, as the mount would hide them. It would be helpful to extend the entry-script to copy over stuff from `/var/roundcube/assets/*` to `/var/www/html/*`. Example: This would allow to privde a logo within the image at `/var/roundcube/assets/public_html/logo.svg`, which would be updated in the mount each time the container starts. Note: This is mostly important for php-fpm use cases. This issue is probably related to #337 and also #396