Permission issue does not allow to create config file

If we install compose in a directory that does not belong to `www-data` (very common when the code is mounted from outside a Docker container), compose cannot create the configuration file for the core package at boot. This prevents the default value of `maintenance_mode` from `metadata.json` to be written, and the user is redirected to `maintenance` since the default value passed to `Core::getSetting('maintenance_mode', 'core')` is `true`.

We either show an error in this case or make it easier for the user to figure out what the issue is. Perhaps, we could use the entrypoint script to check and notify the user somehow (on-the-fly written page?).