Paywall: error 500 when I add a website

Version: 2.6.14
Installation: Installed to Docker on both Synology and UGreen NAS, with Portainer.
PHP version: NSP
OS: Synology DSM 7 / UGreen OS
Database: MariaDB (the one install with the standard deployment script)
Parameters:

```
services:
wallabag:
image: wallabag/wallabag
container_name: Wallabag
restart: on-failure:5
environment:
- MYSQL_ROOT_PASSWORD=wallaroot
- SYMFONY__ENV__DATABASE_DRIVER=pdo_mysql
- SYMFONY__ENV__DATABASE_HOST=db
- SYMFONY__ENV__DATABASE_PORT=3306
- SYMFONY__ENV__DATABASE_NAME=wallabag
- SYMFONY__ENV__DATABASE_USER=wallabag
- SYMFONY__ENV__DATABASE_PASSWORD=wallapass
- SYMFONY__ENV__DATABASE_CHARSET=utf8mb4
- SYMFONY__ENV__DOMAIN_NAME=http://xxx.xxx.xxx.xxx:6749
- SYMFONY__ENV__SERVER_NAME=xxxxxxxxx
- SYMFONY__ENV__FOSUSER_CONFIRMATION=false
- SYMFONY__ENV__FOSUSER_REGISTRATION=true
- SYMFONY__ENV__TWOFACTOR_AUTH=true
ports:
- 6749:80
volumes:
- /volume1/docker/wallabag/images:/var/www/wallabag/web/assets/images:rw
- /volume1/docker/wallabag/data:/var/www/wallabag/data:rw
depends_on:
- db
- redis
db:
image: mariadb:11.4-noble #LTS Long Time Support Until May 29, 2029.
container_name: Wallabag-DB
restart: always
environment:
- MYSQL_ROOT_PASSWORD=wallaroot
- TZ=Europe/Paris
volumes:
- /volume1/docker/wallabag/db:/var/lib/mysql:rw

redis:
image: redis:alpine
container_name: Wallabag-REDIS
healthcheck:
test: ["CMD-SHELL", "redis-cli ping || exit 1"]
volumes:
- /volume1/docker/wallabag/redis:/data:rw
restart: on-failure:5
```

I tried to fix permissions error (chmod 664) but still the same. (I follow explanation given for the similar issue "Paywall: error 500 when I add a website wallabag/wallabag#3477" reported as "closed").
NB : the file "site-credentials-secret-key.txt" wasn't created during program installation nor after having activated the paywall parameter (set to '1'). I created myself with command line and ensure it was then created in data directory.

Start by reproducing the website addition in the supplied Docker Compose deployment and inspect the container error output, then compare the behavior with #3477. Check the presence and permissions of data/site-credentials-secret-key.txt; done means adding a website no longer returns HTTP 500 on the reported MariaDB/Redis setup.

Written by the indexing model from the issue text.