feat(docker/kumactl): make entrypoint consistent

[bartsmykla]

Make kumactl container image entrypoint consistent with kuma-cp and kuma-dp images, where names of images refer to binaries set in entrypoint.

This PR is a follow-up after the discussion in #6593 (comment)

Checklist prior to review

• Link to relevant issue as well as docs and UI issues -- fix(docker/kumactl): add entrypoint to kumactl img #6593 (comment)
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS -- it doesn't
• Tests (Unit test, E2E tests, manual test on universal and k8s) -- tests are introduced in another PR: ci(tests): add container structure tests #6581
• Do you need to update UPGRADE.md? -- I updated the file
• Does it need to be backported according to the backporting policy? -- it doesn't
• Do you need to explicitly set a > Changelog: entry here or add a ci/ label to run fewer/more tests?

Changelog: feat(docker/kumactl): make entrypoint consistent with kuma-cp and kuma-dp images

[michaelbeaumont]

LGTM!

[lahabana]

I think this is changing the behaviour which requires changing all the docs and anyone using any script for little added value on our side.
What's the motivation behind this?

[bartsmykla]

I think this is changing the behaviour which requires changing all the docs and anyone using any script for little added value on our side. What's the motivation behind this?

I think motivation is consistency. To be honest I'm not convinced that it would impact that much anyone using any script, as this was broken from 2.1.0, and so far there was only one complain about it. I fixed and backported the change in the other PR. I looked in the documentation and there is only one place where we are mentioning using kumactl image, and it was removed from the latest docs (ref. https://github.com/kumahq/kuma-website/blob/f06a058a0f28d2186289cbf25ef727649cb57e0f/app/_src/installation/docker.md#L94)

[michaelbeaumont]

Do we need to update the Helm charts? I wanna take a look

[michaelbeaumont]

I think we might need to set command in

kuma/deployments/charts/kuma/templates/pre-upgrade-install-crds-job.yaml

Lines 163 to 164 in 4f2f48f

	args:
	- '/kuma/scripts/save_crds.sh'

command: ["/busybox/busybox", "sh", "-c"]

@lahabana I think for _ctl docker images or even any docker image, the user's assumption will be that the entrypoint is the binary.

[bartsmykla]

@michaelbeaumont I updated this hook, but I went with sh -c instead of explicit busybox as I believe it's safer for other charts which have this one as a dependency

[michaelbeaumont]

LGTM at least

[bartsmykla]

@lahabana this also makes it container structure tests more reusable as without this, we couldn't test entrypoint for kumactl image if we want ubi images to reuse the same tests as ubi images don't use busybox

[bartsmykla]

We did internal poll about this, and decided to introduce this change