docker-compose.yml

version: "3.5"

services:
  dind: # Docker-in-Docker service to allow nested Docker usage
    image: docker:20-dind # Use the Docker version 20 image with Docker-in-Docker enabled
    restart: always # Ensure the service is always restarted in case of failure
    privileged: true # Run the container in privileged mode to allow Docker daemon to run inside
    environment:
      DOCKER_TLS_CERTDIR: "" # Disable TLS for Docker daemon
    command:
      - --storage-driver=overlay2 # Use overlay2 storage driver for Docker

  runner: # GitLab Runner service to execute CI/CD jobs
    restart: always # Ensure the service is always restarted in case of failure
    image: registry.gitlab.com/gitlab-org/gitlab-runner:alpine # Use the alpine-based GitLab Runner image
    depends_on:
      - dind # Ensure dind service is started before this service
    environment:
      - DOCKER_HOST=tcp://dind:2375 # Set Docker host to use Docker-in-Docker service
    volumes:
      - ./config:/etc/gitlab-runner:z # Mount local config directory to the GitLab Runner configuration directory

  register-runner: # One-time service to register the GitLab Runner
    restart: "no" # Do not restart the service after it completes
    image: registry.gitlab.com/gitlab-org/gitlab-runner:alpine # Use the alpine-based GitLab Runner image
    depends_on:
      - dind # Ensure dind service is started before this service
    environment:
      - CI_SERVER_URL=${CI_SERVER_URL} # GitLab CI server URL (set through environment variable)
      - REGISTRATION_TOKEN=${REGISTRATION_TOKEN} # Registration token for GitLab Runner (set through environment variable)
    command: # Command to register the runner with the provided parameters
      - register
      - --non-interactive # Run the registration in non-interactive mode
      - --locked=false # Allow the runner to pick up any available jobs
      - --name=${RUNNER_NAME} # Set the runner name (from environment variable)
      - --executor=docker # Use Docker executor
      - --docker-image=docker:20-dind # Use Docker-in-Docker image for running jobs
      - --docker-volumes=/var/run/docker.sock:/var/run/docker.sock # Mount Docker socket for Docker-in-Docker
    volumes:
      - ./config:/etc/gitlab-runner:z # Mount local config directory to the GitLab Runner configuration directory