[BUG] Improve documentation to explain and extend Kanister logical tool container

[mlavi]

Describe the bug
The default container image sourced in many Kanister blueprints is currently ghcr.io/kanisterio/kanister-kubectl-1.18:0.114.0

Here's the Dockerfile: https://github.com/kanisterio/kanister/blob/master/docker/kanister-kubectl/Dockerfile
which I believe uses this Dockerfile for the FROM: definition:
https://github.com/kanisterio/kanister/blob/master/docker/tools/Dockerfile

Observe the binaries/packages added to Red Hat UBI Minimal image userland: Kopia, Restic, gosu (not sure what that is), and kubectl is layered on top from the original Dockerfile.

Kanister used to build containers with more tooling, but it lead to a larger attack surface and shipping CVEs with releases, so we have been forced minimize. It would be best to explain and/or show examples of adding logical tooling is needed by the application backed up by the blueprint.

To Reproduce
https://docs.kanister.io/tooling.html#docker-image

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