Is it still possible to use a Docker deployment that uses an external MariaDB database in version 12?
This is clearly described in version 11, but not mentioned in version 12, or I am not understanding the article correctly.
Or does this violate the principle of containerization?
Yes, it is possible. I had to modify my MariaDB config to allow connections from the network since the container has another IP so it looks like it is coming from another machine.
Details on how to configure your docker seafile to use your existing MariaDB instance can be found in this document.
https://manual.seafile.com/12.0/setup/setup_with_an_existing_mysql_server/