The way it works is that the `getstarted` app has a specific `database.js` config file that will use the `DB` environment variable to setup the right database connection. You can look at the code [here](./config/database.js)
They might be coming from a conflict between a locally running database instance and the docker instance. To avoid the errors either shutdown your local database instance or change the ports in the `./config/database.js` and the `docker-compose.dev.yml` file.