Make the services auto-upgradeable

With some easy modifications on [docker-compose.yml](https://github.com/Steccas/xbs-api-docker/blob/master/docker-compose.yml) and the [dockerfile](https://github.com/Steccas/xbs-api-docker/blob/master/dockerfile) it is possible to fetching automatically the latest version of xbs-api and the other needed components without having to edit the version manually.