I had to go full Rube Goldberg to clean up old image tags from closed PRs, while still leaving deletion of untagged image to the ECR repo's own lifecycle policy. Never go full Rube Goldberg:

• https://stackoverflow.com/questions/70065254/remove-ecr-image-tag-despite-imagereferencedbymanifestlist-error
• https://github.com/aws/containers-roadmap/issues/1567

name: ECR Retention Policy

on:
  pull_request:
    types:
      - closed
  workflow_call:
  workflow_dispatch:

jobs:
  clean-unused-ecr:
    name: Delete unused container images
    runs-on: runs-on,runner=2cpu-linux-x64,run-id=${{ github.run_id }},image=ecr_login_image
    steps:
      - name: Configure AWS credentials
        uses: aws-actions/configure-aws-credentials@v4
        with:
          aws-region: ${{ env.RUNS_ON_AWS_REGION }}
      - name: AWS ECR Login
        id: login-ecr
        uses: aws-actions/amazon-ecr-login@v2
      - name: AWS ECR Info
        shell: bash
        run: |
          echo "ECR_REGISTRY=${{ steps.login-ecr.outputs.registry }}" >> $GITHUB_ENV
          echo "ECR_REPO=$(basename ${{ github.repository }})" >> $GITHUB_ENV
      - name: Docker meta
        id: docker_meta
        uses: docker/metadata-action@v5
        with:
          images: ${{ env.ECR_REGISTRY }}/${{ env.ECR_REPO }}
          flavor: suffix=-
          tags: type=raw,value=${{ github.head_ref || github.ref_name }}
      # NOTE: This is convoluted because AWS ECR has no simple way to untag image without deletion
      # given we want to leave deletion of untagged image to the ECR repo's own lifecycle policy
      # https://stackoverflow.com/questions/70065254/remove-ecr-image-tag-despite-imagereferencedbymanifestlist-error
      # https://github.com/aws/containers-roadmap/issues/1567
      - name: AWS ECR Cleanup
        shell: bash
        run: |
          REPO_EXISTS=$(aws ecr describe-repositories --repository-names $ECR_REPO 2>&1 || true)
          if echo "${REPO_EXISTS}" | grep -q 'RepositoryNotFoundException'; then
            echo "Repository not found, skipping cleanup."
            exit 0
          fi
          IMAGE_TAGS=$(aws ecr list-images --repository-name $ECR_REPO --query 'imageIds[*].imageTag' --output text)

          docker pull busybox
          docker tag busybox $ECR_REGISTRY/$ECR_REPO:_
          docker push $ECR_REGISTRY/$ECR_REPO:_

          TEMP_IMAGE=$(
            aws ecr batch-get-image \
                --repository-name $ECR_REPO \
                --image-ids imageTag=_ )
          TEMP_MANIFEST=$(echo $TEMP_IMAGE | jq -r '.images[].imageManifest')
          TEMP_DIGEST=$(echo $TEMP_IMAGE | jq -r '.images[].imageId.imageDigest')

          TAG_PREFIX=$(echo ${{ fromJSON(steps.docker_meta.outputs.json).tags[0] }} | cut -d: -f2)
          for TAG in $IMAGE_TAGS
          do
            if [[ $TAG == $TAG_PREFIX* ]]; then
              docker tag busybox $ECR_REGISTRY/$ECR_REPO:$TAG
              docker push $ECR_REGISTRY/$ECR_REPO:$TAG
              echo "Untaged image $TAG"
            fi
          done

          # Delete the temporary image by digest
          aws ecr batch-delete-image \
            --repository-name $ECR_REPO \
            --image-ids imageDigest=$TEMP_DIGEST