Custom value for containers' image

[ymitsos]

We have a use case where we need to provide a custom value for the containers' image attribute. Instead of gcr.io/tekton-releases/github.com/tektoncd/pipeline/cmd/webhook we need to define our own repository.

Currently, the existing templates don't facilitate this. Would a PR that will leverage this enhancement go through?

[abelnieva]

#30 drafted this PR allowing setup custom images and default registry