Does KNative serving not allow initContainers field?
Init containers are not there in the Knative serving API spec which leaves me to believe that it's not supported.
To add a little more color to Arghya's answer, Knative deliberately excludes init containers right now because they can induce large (unbounded) amounts of additional latency during cold-start. Additionally, one of the goals for Knative was to simplify much of the Kubernetes API, so solutions like init containers which can also be implemented as startup parts of the user container were blocked off.
One reason for being restrictive here is that it's much easier to add to an API later than to remove from it, so the initial API was minimal to see which constraints were substantially onerous.