Sorry for the confusion.
We are working on packaging phpMyAdmin (so you can do
bitnami-pkg install), which would make it a lot easier to include it in a docker-compose installation, but it's not ready yet.
In the meantime, the easiest and quickest way would be to install the official phpMyAdmin container, as @jorgemarin suggested in GitHub, and configure it to connect to your MariaDB container.
Let us know if you have any question related to this.