Define $COMPOSE_FILE in config to fix abra app deploy error

By default, the config options provided in the template fail because $COMPOSE_FILE is not set, leading to an error `[abra recipe path] is a directory` when running `abra app deploy`. This is fixed by setting `$COMPOSE_FILE` to `compose.yml`

.env.sample

@@ -3,6 +3,8 @@ TYPE=vaultwarden
 DOMAIN=vaultwarden.example.com
 LETS_ENCRYPT_ENV=production
 
+COMPOSE_FILE="compose.yml"
+
 WEBSOCKET_ENABLED=true
 SIGNUPS_ALLOWED=true