A lambda for cleaning out dangling and irrelevant images from the AWS ECR

This JavaScript file can be uploaded into AWS Lambda and used to clean up your ECR repository.

A few things are worth calling out though:

• Right now this is tuned for the organization I work at; you'll probably want to adjust some of the criteria before running it on your own repos (for example, the string 'PR' might not be what you want to look for)
• This assumes that images that are missing a tag are implicitly dangling images. Since AWS wraps the Docker APIs, this is a loose constraint, whereas the Docker API has a native identification of an image being considered dangling (i.e. this could change in later versions of ECS)
• This script keeps the latest 50 images that my organization considers releases. The ECR API fetches 100 at a time, and we're not doing pagination since the assumption is that this runs often enough that there aren't more than 100 images in a single pass.
• The batchDeleteImage call DOES NOT have a dryRun flag. If you're going to use this I strongly urge you to comment out the call here and check the logging this will generated to make sure things look correct.
• Since we're using timestamps at my organization, I had to dig into the metadata a bit to find the created date (it surprised me that this didn't appear to be a first class citizen). The v1Compatibility object seemed to be the only place I could find this info. If you're reading this and know of another, feel free to open an issue or a PR to fix it.

Getting the policy correct for this was a bizarre process, namely because ecr.describeRepositories didn't seem happy with any resource in the policy other than '*'. Trying other ARNs like 'aws:ecr:*' and others suggested in the AWS docs didn't seem to work correctly.

To avoid others going through the same pain, here's the policy I ended up using:

{
  "Version":"2012-10-17",
  "Statement":[
    {
      "Action":[
        "logs:CreateLogGroup",
        "logs:CreateLogStream",
        "logs:PutLogEvents"
      ],
      "Resource":"arn:aws:logs:*:*:*",
      "Effect":"Allow"
    },
    {
      "Action":[
        "ecr:DescribeRepositories",
        "ecr:ListImages",
        "ecr:BatchGetImage",
        "ecr:BatchDeleteImage"
      ],
      "Resource":"*",
      "Effect":"Allow"
    }
  ]
}