Kubernetes source docker files

Question

Trying to understand how a cluster is being setup and how different components of the cluster work together, I am looking into different Kubernetes yaml files in particular the kubeapi-server. There I can see that the image is coming from:

kube-apiserver:
    Container ID:  docker://fac3b6468d5d60c0a5154ac1659d447255467d951522ce6d82c6b9438a6e5c89
    Image:         k8s.gcr.io/kube-apiserver:v1.21.2
    Image ID:      docker-pullable://k8s.gcr.io/kube-apiserver@sha256:c86c3855e360b1483008c30c8deaed2b1a92f63eaacec819a90a0ffe04df152b

So, I know the image itself is stored on k8s.gcr.io and I can reach that but I cannot see the source code for the actual Dockerfile while I assume it should be open source and available in some repo, like for example nginx and alpine where the link to their github is mentioned on the docker hub:

• https://hub.docker.com/_/nginx?tab=description
• https://hub.docker.com/_/alpine

I hope someone knows where to find the Kubeapi-server and other kube* docker image source files.

Update 1: I did some more investigation on the locally stored docker image on my machine to check the base image etc like below:

docker image ls | grep apiserver              
k8s.gcr.io/kube-apiserver                                                                        v1.21.2      106ff58d4308   6 months ago    126MB

docker image history 106ff58d4308 

.... (big output but shows the some info about layers and ENTRYPOINT which points to a base image name)

But what I want is still the Dockerfile source code.