support a docker flavor meant to be used as base image for container-based CI jobs

[osher]

Fixes #2325 in case of a bug fix, this should point to a bug and any other related issue(s)

Description

kaniko:debug is used a lot as a base image for CI job-containers.
However, unintuitively, it requires to use the kaniko:debug image, and to override the entrypoint, plus - it comes with just /busybox, and is therefore very limiting.

This MR is the result of my adventure, as documented here:
https://stackoverflow.com/questions/74402431/adding-build-tools-to-a-kaniko-image-for-gitlab-ci/74402843#74402843

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:

• The project will publish a new flavor of docker image - kaniko:ci, meant to be used more naturally as a base image for jobs on container-based CI setups like Gitlab-CI, and probably circle-ci and github-actions.

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[osher]

I know of one issue - the way I solved it in my project is for a concrete architecture.
If you could point the way on how to do it better - I will :)

keep up the great work.

[osher]

Uh, sorry - I missed the thing about CLA. I did it now...

[osher]

eh..? anybody home?

[osher]

maybe I need to create an issue first - so I have...
#2325

[osher]

im still here, when you're ready

[osher]

@tejal29 @imjasonh - I mention you because I see you're contributors from google-cloud build...

Is the project dead?

[osher]

🎶 I wont let the sun come down on me 🎶

[osher]

😳

[osher]

👀

[gabyx]

I dont think this should be part of any kaniko ci container.
I also wanted to install bash but thats just a little bit hard for this image. Normally in CI you prepare everythin with other containers and then you mount your stuff inside kaniko to do the very rest.
Bash would however be nice to at least be able to write some scripts (not ins ash style...)

[gabyx]

Maybe there is a way to be able to extend the kaniko image in a way programatically by other users, everybody has a different need. That would be nice...