502 bad gateway due to webserver not listening to IPv4 addresses

I have deployed the branch with changes from #211 (closed) on dokku, but unfortunately I failed to test the changes. The service is running properly on port :5000 (checked app logs, tested locally with curl), but we get 502 Bad Gateway when trying to access https://scriptum.apps.ecobytes.net.

The reason for the 502 seems to be that the local server is only listening on ipv6:

$ dokku enter dbp2016-live
root@e77c188b082f:/usr/src/app# netstat -tulpen
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       User       Inode       PID/Program name
tcp6       0      0 ::1:5000                :::*                    LISTEN      0          38531313    -

cc @almereyda