There is still no default ECR Lifecycle policy template or something. So, as you mentioned, you may use aws cli way, and assign this to execute from somewhere, like Lambda, or k8s job:

1. Get all repositories names:

   repositories=($(aws ecr describe-repositories --profile=$profile --output text --query "repositories[*].repositoryName"))
   

2. Apply policy to each repository:

   for repository in "${repositories[@]}";
   do
   aws ecr put-lifecycle-policy --profile=$profile --repository-name $repository --lifecycle-policy-text "file://policy.json"
   done;
   

you can use Terraform for that

resource "aws_ecr_lifecycle_policy" "untagged_removal_policy" {
count      = "${length(split(",",local.registries))}"
depends_on = [ "aws_ecr_repository.ecr_repositories" ]
repository = "${aws_ecr_repository.ecr_repositories.*.name[count.index]}"

policy = <<EOF
{
"rules": [
    {
        "rulePriority": 1,
        "description": "Expire Docker images older than 7 days",
        "selection": {
            "tagStatus": "untagged",
            "countType": "sinceImagePushed",
            "countUnit": "days",
            "countNumber": 7
        },
        "action": {
            "type": "expire"
        }
    }
]
}
EOF

}

I'm using CloudFormation mapping to define one policy and then apply it on all repositories with one line:

Mappings:
 ECRPolicy:
  DevPolicy:
    RemoveUntagged: |
      {
        "rules": [
          {
            "rulePriority": 1,
            "description": "Expire images older than 3 days",
            "selection": {
              "tagStatus": "untagged",
              "countType": "sinceImagePushed",
              "countUnit": "days",
              "countNumber": 3
            },
            "action": {
              "type": "expire"
            }
          }
        ]
      }

And for the repos it's just:

  ECRRepository:
   Type: AWS::ECR::Repository
   Properties:
    RepositoryName: !Sub ${ECRRepositoryName}-dev
    RepositoryPolicyText:
      Version: "2012-10-17"
      Statement:
        - Effect: Allow
          Action:
            - ecr:GetAuthorizationToken
            - ecr:BatchCheckLayerAvailability
            - ecr:GetDownloadUrlForLayer
            - ecr:GetRepositoryPolicy
            - ecr:DescribeRepositories
            - ecr:ListImages
            - ecr:DescribeImages
            - ecr:BatchGetImage
          Principal:
            AWS:
              - !Sub arn:aws:iam::${DevAccount}:root
          Sid: AllowCrossAccountPull
    LifecyclePolicy:
      LifecyclePolicyText: !FindInMap [ECRPolicy, DevPolicy, RemoveUntagged]