Unable to create create docker access object (Connection refused) using GitLab Runner / docker-maven-plugin

I want to build a project using a GitLab runner. That project uses the docker-maven-plugin. The Runner is configured to use privileged mode as advised by GitLab for Docker-in-Docker.

I am able to build the project locally on my computer both under Windows and Mac OS.

The relevant part of the .gitlab-ci.yml looks as follows:

stages:
  - build

variables:
  DOCKER_HOST: tcp://docker:2375

services:
  - docker:dind

Validate:
  image: cschockaert/docker-npm-maven:latest
  stage: build
  tags: 
    - dind
  script:
    - mvn clean install 

However, the GitLab runner yields the following error message:

[ERROR] DOCKER> Cannot create docker access object  [Cannot extract API version from server http://localhost:2375 : Connect to localhost:2375 [localhost/127.0.0.1] failed: Connection refused (Connection refused)]

I know that when building my project locally, I have to activate the option to listen on port 2375 on Windows or, alternatively, run socat TCP-LISTEN:2375,reuseaddr,fork UNIX-CONNECT:/var/run/docker.sock on Mac OS as advised in this thread. To my knowledge, setting DOCKER_HOST: tcp://docker:2375 should have the same effect on the GitLab runner. Replacing it by DOCKER_HOST: tcp://localhost:2375 leads to the same results.

I also checked that DOCKER_HOST is really set by running env | grep DOCKER as advised by this answer.

Source: StackOverflow