Hey Sebastian,
I've been checking with Clement and this might be due to OpenShift not allowing that base image for source builds.
It seems like that to do S2I you need a base image with a certain user (I think that's 1001) and neither the java/ nor the fabric8/ ones do that. Clement mentioned redhat-openjdk-18/openjdk18-openshift images might do that but those I think are behind VPN or require some for of login. Clement also mentioned this might work with minishift, but I've not tried yet.
The alternative might be to switch that example to use binary builds and adjust instructions for OpenShift and plain Kubernetes.
Cheers,
Galder