enhances to run with `tariff-docker`

- Add a `backend_ip.sh` initialiser to container so it adds backend development ip *tariff-api.dev.gov.uk* to `/etc/hosts`
- `database.yml` will need to reference other `ENV` variables when linking containers from `tariff-docker`

For more info see [bitzesty/tariff-docker](https://github.com/bitzesty/tariff-docker)