Define default PUBLIC_FILES_PATH environment variable

[tormi]

Changes

Replacing default file storage with Google's Filestore volume requires redefining PUBLIC_FILES_PATH because it disappears from Silta environment after disabling public-files mount (it follows the mount's naming automatically).

Let's add PUBLIC_FILES_PATH: /app/web/sites/default/files as a default value to our project template to reduce the risks of not having it defined.

cc: @Jancis