As Soma I already enabled 4-Byte support in the past. For the restore of the database I used the "original" script, i.e. not the enhanced command.
Emojis are still fine in nextcloud->mail: Subject and content with emojis are represented correctly. A text file containing emojis looks correct but not in the sidebar but i'm not sure whether this effect is a consequence of the former restore command.
linuxserver/docker-mariadb is migrating away from mariabionic (with 10.1.47) to alpine-10.5.8 (see https://github.com/linuxserver/docker-mariadb/releases).