add support for initContainers about k8s-sidecar-injector

[iwilltry42]

@komapa I couldn't find any resource regarding your concerns. Only a wrong order of admission controllers might prevent injecting containers, e.g. PodSecurityPolicies (but containers in general, not only initContainers).
Anyways I can confirm for #8, that it works.
I tested it on Minikube and in our development cluster (though we don't have stuff like PodSecurityPolicies there).

[komapa]

I thought initContainers cannot be injected?

[byxorna]

@komapa i am going to reinvestigate when i have time, and confirm. it would be great if we could; if not, ill have links here explaining exactly why they cant be patched into a pod.

[byxorna]

@komapa @iwilltry42 I initially looked into this (way before this was opensourced) and I thought I found some reason I couldnt inject initContainers; I was clearly very mistaken :)

Thanks for sorting this out!

[byxorna]

Closed via #8