Build debug-agent image from distroless static image

[tkanng]

Hi aylei, this pr might be the solution for #19. I've tested it in my cluster, distoless image prevents user executing shell in the agent container, just like this:

root@iZhp37kmiszbkwzt5oh9csZ:~/k/kube-debug# kubectl get pod
NAME                    READY   STATUS    RESTARTS   AGE
debug-agent-t8sgd       1/1     Running   0          17m
debug-agent-wfvp4       1/1     Running   0          17m
hhhh-5f88b9f6bc-qrwrm   1/1     Running   0          3d
nginx                   1/1     Running   0          3d2h
root@iZhp37kmiszbkwzt5oh9csZ:~/k/kube-debug# kubectl exec -it debug-agent-t8sgd sh
OCI runtime exec failed: exec failed: container_linux.go:345: starting container process caused "exec: \"sh\": executable file not found in $PATH": unknown
command terminated with exit code 126

[aylei]

Awesome! I'm afraid I don't have time to do a manual test now, but hopefully I will test this PR and give feedbacks in two days.

[tkanng]

Thanks！ :)

[aylei]

Thank you!