oh, I see what you mean!

they do that for the sake of providing an example that works instantly. but on the long term it's not a good idea. if you intend to keep using a service, you are better off connecting it to a postgres db that's shared across all services. once you get used to it, you'll do that even for those services that you are just quicly trying out.

how I do this is I have a separate docker compose that runs a postgres and a mariadb. and these are attached to such a docker network, which is created once with a command, rather than in a compose file. every compose file where the databases are needed, this network is specified as an "external" network. this way containers across separate compose files can communicate.
my advice is its best to also have this network as "internal" too, which is a weird name but gist is, this network in itself won't provide access to your LAN or the internet, while other networks may still do that if you want.

basically setup is a simple command like "docker network create something something", and then like 3 lines in each compose file. you would also need to transfer the data from the separate postgreses to a central one, but thats a one time process.

let me know if you are interested, and I'll help with commands and what you need. I don't mind it either if you only get around to this months later, it's fine! just reply or send a message