How can I generate multiple Strapi databases?

[Eventyret]

@AndyDMC

However, the problems start when I try to build a second site, still using the strapi-data-test volume for the MariaDB container. I modified the .env file so it used a different db name and username:

You will need to either create a separate DATABASE (easiest method) so run 2 strapi instances and 2 databases)
Else you have to create a init script for mariadb to create the other tables.
Here is an example of it

Docker Compose MariaDB Multiple Database

Using the multiple tables, you can then change the variables for each one to connect to a separate table and when it starts it creates them.