feat: configure Drupal file paths in Nginx Dockerfile

[esolitos]

What

This change sets up the default environment variables and file paths for a Drupal application running in an Nginx container. It creates a symlink to the default location where the persistent files are stored, which is usually done at runtime by calling the makefiles script. This change ensures that the file paths are properly configured without having to run the script every time the container is started.

Why

Currently we create the symlink using the make -f /usr/local/bin/actions.mk init command when we start the container, which calls the init script and in our scenario does the same thing as we are doing here by calling the files_link command.

This is not great as it requires an additional step at deploy time, and since we always use the same setup we can hardcode this at build time instead of at run time.

Additional info

This is the nginx companion of previous PRs: #7