refactor check for non-empty workdir

introduce new envvar `ROUNDCUBEMAIL_ALLOW_DIRTY_DOCROOT`, which can be truish, falsish or empty: | value | result | |---|---| | `true` | skip checks for empty docroot | | `false` | abort if docroot is not empty | | (empty) | if the docroot is not empty, print a warning and sleep, giving the user the chance to manually abort. | this keeps backwards compatibility with existing deployments (that do not have the envvar set) the sleep timeout has been raise to 60seconds, so the user has more time to manually abort (in an autostart context, this will add an additional delay, but shouldn't actually be harmful) - Closes: https://github.com/roundcube/roundcubemail-docker/issues/384 - obsoletes: https://github.com/roundcube/roundcubemail-docker/pull/387