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description Tutorial on how to test if Docker-in-Docker service in GitLab CI accesses Docker Hub directly and how to configure pull-through cache to an Kubernetes cluster working as a GitLab runner.
tags
Docker
GitLab CI

Docker-in-Docker service and Docker Hub ratelimit

The recommended way of running Docker commands in a GitLab CI Kubernets runner is to use a service to run Docker-in-Docker container (docker:dind). By default the docker:dind container pulls container images from Docker Hub without caching. This can cause problems as pulls from Docker Hub are rate-limited.

This tutorial tests how docker pull ubuntu:24.04 commands consume Docker Hub ratelimit and provides example on how to configure pull-through cache to an Kubernetes cluster working as a GitLab runner.

How to check if docker pull commands are cached

Create a new GitLab project with .gitlab-ci.yml and scripts/check-docker-hub-ratelimit.sh files.

The scripts/check-docker-hub-ratelimit.sh script prints current Docker Hub ratelimit and also writes the result to a file if filename is given as first parameter. See Docker Hub usage and rate limits article in Docker documentation for more details.

---8<--- "docs/tutorials/gitlab-ci/docker-hub-ratelimit/scripts/check-docker-hub-ratelimit.sh"

The pipeline defined by .gitlab-ci.yml tries to pull the same Docker image twice and checks if the rate limit headers are different after first and second pull.

---8<--- "docs/tutorials/gitlab-ci/docker-hub-ratelimit/.gitlab-ci.yml"

The pipeline should fail, if images are pulled directly from Docker Hub.

How to setup pull-through cache to Kubernetes runner

Manifests for configuring pull-through cache and configmap are available in the repository that provides this website.

git clone https://github.com/cicd-tutorials/cicd-tutorials.net.git
cd docs/tutorials/gitlab-ci/docker-hub-ratelimit

Configure pull-through cache and configmap for GitLab runner by running kubectl apply -f manifests/.

kubectl apply -f manifests/

These manifests assume that GitLab runner is using gitlab-runner namespace. Edit the namespace value in docker-config.yaml if this is not the case.

Modify the GitLab runner configuration so that the configmap defined in docker-config.yaml is mounted to all containers launched by the GitLab runner. Example of a full values.yaml help input file below.

gitlabUrl: # Your instance URL
rbac: { create: true }
runnerToken: # Your runner token
runners:
  config: |
    [[runners]]
      executor = "kubernetes"
      [runners.kubernetes]
        image = "alpine:3.12"
        privileged = true
        [[runners.kubernetes.volumes.config_map]]
          name = "dockerd-config"
          mount_path = "/etc/docker/daemon.json"
          sub_path = "daemon.json"