Proper docker configuration for both gitlab & a runner running on local docker containers

I am trying to get a docker compose file to run a local gitlab + gitlab-runner and it seems I have a naming / network issue.

What IS working :

• I have gitlab and the gitlab runner up and running
• gitlab works as a remote for local repos, cloning, pushing & so on.
• gitlab-runner works fine as a runner for my projects on other gitlab servers (tested on gitlab.com)
• I can register the runner for a project in my gitlab container, view the config, trigger a job (the docker executor starts and gets the image to run the environment)

What IS NOT working :

• The job fails when trying to get source from the project repository with the following error message (the repository url it tries to access is correct)
  > fatal: unable to access 'http://gitlab:8081/cours/pipelinediscover.git/': Failed to connect to gitlab port 8081 after 131012 ms: Couldn't connect to server

Here are

1. My current docker compose yml

version: '3.6'
services:
  gitlab:
    image: 'gitlab/gitlab-ce:latest'
    restart: always
    container_name: gitlab
    hostname: 'gitlab'
    environment:
      GITLAB_OMNIBUS_CONFIG: 
        external_url 'http://gitlab:8081'
        # Add any other gitlab.rb configuration here, each on its own line
    ports:
      - '8081:8081'
      - '4443:443'
      - '2222:22'
    volumes:
      - './config:/etc/gitlab'
      - './logs:/var/log/gitlab'
      - './data:/var/opt/gitlab'
    shm_size: '1024m'
  runner:
    image: 'gitlab/gitlab-runner:latest'
    restart: always
    volumes:
      - './gitlab-runner/config:/etc/gitlab-runner'
      - '/var/run/docker.sock:/var/run/docker.sock'
    container_name: gitlab-runner

n.b I had to edit my host /etc/hosts file to be able to access the interface at gitlab:8081 and not localhost:8081 that may not be ideal but if gitlab is configured with external_url at "localhost:8081" then the gitlab-runner cannot interface with it at all. There might be some sweet spot in both docker & gitlab config I missed for domain/network naming.

2. The runner toml config :

concurrent = 5
check_interval = 0
shutdown_timeout = 0
[session_server]
  session_timeout = 1800
[[runners]]
  name = "05f411d789cb"
  url = "http://gitlab:8081/"
  id = 6
  token = "9HXz2_XQzLCpqPiTqDUZ"
  token_obtained_at = 2023-03-31T07:09:00Z
  token_expires_at = 0001-01-01T00:00:00Z
  executor = "docker"
  [runners.cache]
    MaxUploadedArchiveSize = 0
  [runners.docker]
    tls_verify = false
    image = "alpine:latest"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    shm_size = 0

It looks like you haven't configured networking between the services in your compose file.

If you configure both services to use the same network (or specify depends_on: [gitlab] for the runner service) the name should be resolvable by default and the connection will be allowed.

services:
  gitlab:
   networks:
     - appnet
  # ...
  runner:
    # ...
    networks:
      - appnet
networks:
  appnet:

Or

services:
  # ...
  runner:
    depends_on: [gitlab]
    # ...