Gitlab Runner Build DinD fails "Fetching changes" from Repo - Connection Refused

I set up a diid build runner which gets triggerd as expected but fails when fetching changes:

Getting source from Git repository 00:01
Fetching changes with git depth set to 20...
Reinitialized existing Git repository in /builds/###/###/###/.git/
fatal: unable to access '': Failed to connect to gitlab.compyn.tld port 443 after 2 ms: Connection refused

First I thought this is a network issue, that the build container can’t reach the gitlab instance but the “connection refused” let’s me think it’s a permission error or the firewall of the gitlab server rejects connections from the build conainer?

So I was wandering whom I must grant access to the repo to let the runner access it? Or with what network/ip the build container tries to access the repo and what I have to let the firewall pass.
Or does is the problem something completely different?

I’m not (yet) that deep into docker and it’s networking to get that info with a blink of an eye and due to thinking and trialing that a lot I’m not getting it straight in my head anymore :wink: .

I’d be really happy for any hints or explanations regarding this.

Gitlab CE 14.9.5
gitlab-runner 15.1.0


image: docker:dind
  DOCKER_HOST: tcp://docker:2376

  - docker:dind

  - docker info

  stage: build
    - export
    - docker login -u <USER> -p <PW>
    - docker build -t .
    - docker push


concurrent = 1
check_interval = 0

  session_timeout = 1800

  name = "build runner"
  url = ""
  token = "#######"
  executor = "docker"
    tls_verify = false
    image = "docker:dind"
    privileged = true
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/certs/client","/cache"]
    shm_size = 0```