Repo2docker for zero to jupyterhub

Hi. I would like to customize a docker image from the existing spicy-notebook and install to my Kubernetes cluster.

There is a tools called repo2docker as mentioned at the below link but I am not so sure about the steps to build from the existing image.

I created a github repository and have written an requirements.txt but where should I indicate it should install base on the existing image?

Below link as reference: