Using private ECR image on docker-deployed gitlab runner

I currently have a runner registered for my group at running natively from the official PPAs on Ubuntu 20.04. This runner connects to a private AWS ECR to pull the docker images to run our pipelines and it works fine.

On the same machine, we are trying to set up a runner using the docker image gitlab/gitlab-runner, but it does not seem to be properly authenticating for the AWS. The pipelines are the same with the DOCKER_AUTH_CONFIG set as a group runner variable and set to:

  "credHelpers": {
    "************": "ecr-login"

I thought maybe it was failing because the gitlab/gitlab-runner instance was missing the amazon-ecr-credential-helper packages in the image, but using a custom image that included this didn’t seem to help.