services: transmission: use secret environment

To ensure that I do not repeat the mistake from last time...

.gitignore

@@ -1,5 +1,6 @@
 /*/*
 !*/*.env
+/*/*.env.secret # Just to document that they should definitely never be added
 
 # This is not necessary with the wildcard used in the first line, but it is clearer
 /letsencrypt/*

docker-compose.yml

@@ -111,6 +111,7 @@ services:
     restart: unless-stopped
     env_file:
       - ./transmission/transmission.env
+      - ./transmission/transmission.env.secret
     volumes:
       - ./transmission:/config
       - ~/downloads:/downloads