Check if scan already is in progress

[pzi]

As per discussion in #3:

• This enables polling if the ECR scanning is already in progress by the time this GitHub action checks for findings.
• Skips the startImageScan if scan is already in progress.

[pzi]

@alexjurkiewicz I haven't used that feature myself yet, but I assume you can just update my PR with the code you got in that branch given I allowed "edits by maintainers"?

We forked the repository and are using your latest commit https://github.com/alexjurkiewicz/ecr-scan-image/commit/286f5f94a5adbd6d3ac6420a28184a795fc54899 successfully in our pipeline. So it should be a straightforward merge when you get to it 👍

Thanks again! Enjoy your weekend & stay safe over there in NSW. Cheers from Perth.