docker4drupal#63 Ensure setting files are writeable and set them read-only afterwards

Undo previous fixes and fix the problem with correct calculation of webroot now that Composer returns the full path and not the relative one

- Introduce a new environment variable DRUPAL_LIVE defaulting to 0
- Overwrite settings files when scaffolding with this plugin in local development environment
- Add ahoy commands to switch between live and development mode