Add GCR compatibility

[nekwar]

Description

This PR includes GCR compatibility for Starboard operator. Basically, what you need to do to make Trivy scan images from GCR is to add GOOGLE_APPLICATION_CREDENTIALS environment variable with value that equals to path to Service Account json file. With this changes you can easily make vulnerability scan jobs actually scan images from GCR.

Related issues

• Close #120 (Initial problem was about GCR)
• Close #1189

Checklist

• [x] I've read the guidelines for contributing to this repository.
• [x] I've added tests that prove my fix is effective or that my feature works.
• [x] I've updated the documentation with the relevant information (if needed).
• [x] I've added usage information (if the PR introduces new options)
• [ ] I've included a "before" and "after" example to the description (if the PR is a user interface change).

[CLAassistant]

All committers have signed the CLA.

[chen-keinan]

@nekwar its look like a very nice contribution however , starboard has been migrated to trivy-operator and it is no longer maintained. it will be great if you could make this contribution to trivy-operator