Support for Docker swarm secrets

Nobody has claimed this yet.

Hi,

Many Docker images now allow reading values such as passwords from a secret file. This is a very useful feature and is supported by MariaDB, for example. I was wondering if Matomo could support the same. As an example of a docker-compose configuration:

```yaml
secrets:
root_password:
file: ./secrets/root_password.txt
maria_password:
file: ./secrets/maria_password.txt
```

I can then make use of this in the MariaDB service

```yaml
secrets:
- root_password
- maria_password
environment:
- "MYSQL_ROOT_PASSWORD_FILE=/run/secrets/root_password"
- "MYSQL_DATABASE=${MYSQL_DATABASE:-matomo}"
- "MYSQL_USER=${MYSQL_USER:-matomo}"
- "MYSQL_PASSWORD_FILE=/run/secrets/maria_password"
```

I would like to be able to do the same for Matomo as follows:

```yaml
secrets:
- maria_password
environment:
- "MATOMO_DATABASE_PASSWORD_FILE=/run/secrets/maria_password"
```

Start by tracing how the Docker image currently reads Matomo database environment variables, then compare that behavior with the Docker Swarm secret-file convention shown in the issue. Verify the proposed MATOMO_DATABASE_PASSWORD_FILE setting with the provided compose configuration and confirm that the database password is read from /run/secrets/maria_password without exposing its contents.

Written by the indexing model from the issue text.