The mailboxes are in /var/lib/vmail in the dovecot container or as root accessible in /home/mail1/.local/share/containers/storage/volumes/dovecot-data/_data/
For similar cases, it could be possible to open the mailbox with IMAP master user credentials and expunge all messages. This approach shouldn’t require the command line and fiddling with absolute paths in the filesystem. See Mail — NS8 documentation.
This “solution” shouldn’t require CLI, agreed. But it does need a working Mail Server, including retrieval method like IMAP, which isn’t yet the case here.
An additional issue is the fact that this server runs as “international”, using a .com official domain.
Mail, on the other hand, will use a national domain. I do not yet have the DNS entries needed to make this working (yet).
Otherwise I could also have used Roundcube to erase all mails and the trashcan of that user…