Build and use image for ./examples/deployment/kubernetes/createtree

After #2754 gets merged, we should change the examples to use a pre-released container, but we can't do this before we actually merge the PR and then cut a release for the newly added binary.

Feel free to add an issue (assigned to me) and TODO here to swap this over to use a pre-built image so that ko isn't needed.

_Originally posted by @mhutchinson in https://github.com/google/trillian/pull/2754#discussion_r909893450_