of course I tried to get a response from you ;)
Q) Why would be a working docker image / docker-compose example the attractive?
A) Let's compare it with for instance Nextcloud installation. That requires a webserver, a database and of course some storage. For sure, mariadb is simple to install directly as is Nextcloud. But in terms of "transportability" from one server to another, or in terms of low level backup, there is a difference. With a proper docker-compose installation making a backup of everything, including every bit of configuration, data etc. it is just a "docker-compose down" and afterwards doing a tar of a single folder.
Bringing this complete installation to another computer, a replaced server etc. is just unpacking the tar and saying "docker-compose up -d" and it is running again.
To update Nextcloud it only takes changing the version number in the compose file an then again docker-compose down / up -d. To take a snapshot one could use docker-compose stop (or pause), make the snapshot (quite fast with zfs) and afterwards compose start (or resume). That's it.
All requirements such as JRE and tomcat and the files of mailarchiva are tied together in one place (the docker image) reducing the number of decisions and chances of misconfiguration of those components.
Supporting/providing examples of a dockerized mailarchiva could IMHO give it a boost in sales.