Unprivileged Stalwart Mail Server user can read files as root

Stalwart Mail Server is an open-source mail server. Prior to version 0.8.0, when using `RUN_AS_USER`, the specified user (and therefore, web interface admins) can read arbitrary files as root. This issue affects admins who have set up to run stalwart with `RUN_AS_USER` who handed out admin credentials to the mail server but expect these to only grant access according to the `RUN_AS_USER` and are attacked where the attackers managed to achieve Arbitrary Code Execution using another vulnerability. Version 0.8.0 contains a patch for the issue.