New rule: AWS::ECR::Repository should have scanOnPush enabled

[james-sorling-wcar]

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ecr-repository.html

MyRepository:
Type: AWS::ECR::Repository
Properties:
RepositoryName: "test-repository"
ImageScanningConfiguration:
scanOnPush: "true"

Amazon ECR image scanning helps in identifying software vulnerabilities in your container images. Amazon ECR uses the Common Vulnerabilities and Exposures (CVEs) database from the open-source Clair project and provides a list of scan findings.

https://docs.aws.amazon.com/AmazonECR/latest/userguide/image-scanning.html