feat: adds kaniko support

[SteveShani]

Pull Request Submission Checklist

• Follows CONTRIBUTING guidelines
• Includes detailed description of changes
• Contains risk assessment (Low | Medium | High)
• Highlights breaking API changes (if applicable)
• Links to automated tests covering new functionality
• Includes manual testing instructions (if necessary)
• Updates relevant GitBook documentation (PR link: ___)

What does this PR do?

Adds support for scanning kaniko images as they aren't always OCI compliant.

Where should the reviewer start?

snyk/snyk-docker-plugin#642

How should this be manually tested?

Create a kaniko image: docker run -v .:/workspace [gcr.io/kaniko-project/executor:latest](http://gcr.io/kaniko-project/executor:latest) --dockerfile /workspace/Dockerfile --destination "my-kaniko:latest" --context dir:///workspace/ --tar-path my-kaniko.tar --no-push

Then scan it with snyk container test kaniko-archive:my-kaniko.tar

Risk Assessment

Low. Is backwards compatible and requires users to manually specify kaniko-archive:.

Automated Tests:

snyk/snyk-docker-plugin#642 (Multiple tests as part of that PR)