build: extend image / chart checking ci to cover values-acc.yaml

Refactors the image / chart checking to:

• make it easier to include additional repositories beyond our teams replication.
• only scan the difference in images from feature branch to master to avoid hitting docker rate limits.

This grew to be somewhat complicated, so I moved out into its own script to make it easier to test / validate locally. To scan the ats replication run ./script/validate_images.sh values-acc.yaml.