Can't get first-time set-up to work

I've been trying to get a docker container set up for the past few hours, but I keep getting the following error message from the immich_server container:
PostgresError: password authentication failed for user "postgres"

Upon checking the immich_postgres logs I find the errors
[104] FATAL: password authentication failed for user "postgres" [104] DETAIL: Role "postgres" does not exist.

I don't really get why this is happening, since I have changed the DB_USERNAME and
DB_PASSWORD
variables in the .env file to custom values, so I don't understand why the postgres user/pass is still used.

This happens when no other Docker containers are running.
I have tried restarting the containers, clearing all volumes and folders that are listed in the compose file, and I have looked through some other discussion on this Discord (there was a recent case of Portainer interfering, but like I said, there are no other containers running).
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