FR: Support Docker secret usage for DB_PASS

Hi, thank you for creating this container. Highly appreciated.
I currently want to create a [coop-cloud recipe](https://recipes.coopcloud.tech/) out of it. It already runs fine, but when I tried to improve the security by using docker secrets for the database secrets, I realized that the container does not support the classic _FILE mechanism. In combination with the S6 overlay I cannot even work around it with an entrypoint script.