Limit ECR user policy to push only

[osvaldot]

The module creates a (optional) IAM user to interact with ECR repositories, often used in external pipelines to push images to ECR, limit the user policy to allow push only on repositories created by the module.

eg:

      {
         "Sid":"ManageRepositoryContents",
         "Effect":"Allow",
         "Action":[
                "ecr:BatchCheckLayerAvailability",
                "ecr:GetDownloadUrlForLayer",
                "ecr:GetRepositoryPolicy",
                "ecr:DescribeRepositories",
                "ecr:ListImages",
                "ecr:DescribeImages",
                "ecr:BatchGetImage",
                "ecr:InitiateLayerUpload",
                "ecr:UploadLayerPart",
                "ecr:CompleteLayerUpload",
                "ecr:PutImage"
         ],
         "Resource":"arn:aws:ecr:us-east-1:123456789012:repository/my-repo"
      }