As Ben suggested I used "docker logs <container-id>" to view the origin-docker-builder log.
Turns out the docker-builder was unable to resolve github.com
as it copies resolv.conf settings from the host, which in my case was set for local domain DNS servers (I'm using a local compiled openshift binary).
Setting the docker host resolv.conf to 184.108.40.206 resolved the issue.
Thanks for your help!