How to use rkt as container runtime instead of docker for kubernetes?
That's the way to run Rktlet. However, Rktlet is still pretty experimental and I believe it's not being actively developed either. The last commit as per this writing was in 05/2018.
You can try running it the other way as described here or here. Basically, use --container-runtime=rkt, --rkt-path=PATH_TO_RKT_BINARY, etc. on the kubelet.
Is there a reason why you are need rkt? Note that --container-runtime=rkt is deprecated in the latest Kubernetes but should still work (1.13 as of this writing).