openbis-docker: Point file-server.repository-path to a sub-folder of the DSS root dir by default

We got a ticket from a user complaining about that using the default DSS service.properties with our openbis-app docker image, the folder pointing to the raw-store is by default NOT located within the openbis-app-data volume. He has fixed this now following what I've recommended in the ticket. Since this is probably a very common problem, I would recommend that we just change the proposed openbis-app.env and specify the appropriate file-server.repository-path parameter therein. The path I've proposed is the same as we use in production, using the internal variable $root_dir, which corresponds to the volume openbis-app-data. Please accept or comment.