Gitlab Runner with Docker Executor Ignoring DOCKER_AUTH_CONFIG

I’ve got my DOCKER_AUTH_CONFIG CI variable set up, in Gitlab.
I’ve got Gitlab Runner started. It’s running in a docker container:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
gitlab/gitlab-runner:latest “/usr/bin/dumb-init …” 47 hours ago Up 27 hours gitlab-runner

I’d expect to see this message:
authenticating with credentials from $docker_auth_config in the pipeline logs, but it looks like this CI variable is completely ignored?

build-job:
stage: build
image: <my_private_repo>:<some_tag>
before_script:
- docker info
script:
- # never reaches here

I cannot figure out why DOCKER_AUTH_CONFIG is being ignored - any ideas?

did you find a resolution? I have been trying to figure out the same today