Feature: debug shell

[kvaps]

Description

This PR adds two new options: --debug and --debug-shell which allow to run interactive shell in case of RUN command failed.
This option might be very useful during the development the Dockerfile. I'm going to implement it's support into kubectl-build plugin.

Live demo:

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:
- kaniko adds a new flag `--debug` and `--debug-cmd` to spawn interactive debug shell in case of RUN command failed

[kvaps]

PR rebased to v1.7.0, new docker images with this change prepared:

ghcr.io/kvaps/kaniko-executor:v1.7.0
ghcr.io/kvaps/kaniko-executor:v1.7.0-debug
ghcr.io/kvaps/kaniko-warmer:v1.7.0

source code of fork in master branch:
https://github.com/kvaps/kaniko/

[tejal29]

rebasing with latest master

[gabyx]

Awespme!

[aaron-prindle]

@kvaps would you be able to rebase the PR here? Would like to get this in if possible, thanks!