feat: add support for no push environment variable

[sachasmart]

Supports #2385 and #144 in case of a bug fix, this should point to a bug and any other related issue(s)

Description

This feature adds support for checking the KANIKO_NO_PUSH environment variable. If the environment variable is declared, it sets the value to opts.NoPush.

Side note - fully aware there may be an opportunity to use viper here. Happy to help implement it, but I was not sure if this was a path the project was interested in.

Submitter Checklist

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• Includes unit tests
• Adds integration tests if needed.

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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.

Add KANIKO_NO_PUSH environment variable to set --no-push flag

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[JeromeJu]

Thanks for this @sachasmart , would you mind also help add this to the Readme.md?

[sachasmart]

Perfect - updated with:

#### Flag `--no-push`

Set this flag if you only want to build the image, without pushing to a
registry. This can also be defined through `KANIKO_NO_PUSH` environment
variable.

Edit: I have prettier formatting my Markdown and it looks like it updated some of the formatting in the README.md - happy to revert it, but it does look like it may be for the better? Notably the table has extra spaces, etc.

VSCode User Config:

...
  "[markdown]": {
    "editor.defaultFormatter": "esbenp.prettier-vscode"
  },
...