FR: Support Docker secret usage for DB_PASS

[stevensting]

Hi, thank you for creating this container. Highly appreciated. I currently want to create a coop-cloud recipe 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.

[dorianim]

Unfortunately, I'm currently too busy to look into this myself. But if you come up with a PR, I'm happy to look at it :)