Add option to exit 1 if unable to connect

[DanielRamosAcosta]

Using docker-compose and starting both mongoku and mongo provokes a permanent error message in the UI. If mongoku exited with code 1 if it's unable to connect to the database, we could add a restart: on-failure and then the next time it will connect.

Is it possible to add an environment variable to exit 1 if it's unable to connect? I can PR if needed.

Thanks you!