How to use exec command with the dockerized version of gitlab-runner

Summary

Trying to debug a failing gitlab ci job by using the docker gitlab-runner exec functionality. But it fails right away by complaining that it cannot find the local git repository:

[project-root]$ docker run --rm -t -i gitlab/gitlab-runner exec docker test
Runtime platform                                    arch=amd64 os=linux pid=6 revision=5a147c92 version=11.11.1
Not a git repository
To compare two paths outside a working tree:
usage: git diff [--no-index] <path> <path>
WARNING: You most probably have uncommitted changes. 
WARNING: These changes will not be tested.         
fatal: Not a git repository (or any of the parent directories): .git
FATAL: exit status 128                             
[project-root]$

But obviously there is a git repo (and .gitlab-ci.yml) in the same directory.

It is the first time that I'm trying to use gitlab-runner locally. So no later or earlier config/caching stuff which could have f*ked it up. Ubuntu 18.04.

Steps to reproduce

docker run --rm -t -i gitlab/gitlab-runner exec docker test

Gitlab ci yaml is not relevant I think because the problem happens before the parser reads the yaml.

.gitlab-ci.yml

variables:
  GIT_SSL_NO_VERIFY: "1"
  APP_ENV: "ci"

image: gitlab.servers.kantoor.eztrack.nl:4567/platform/laguna:latest

stages:
  - test
  - build
  - trigger integration test
  - deploy dev
  - deploy test

test:
  stage: test
  services:
    - mysql:5.7
    - mongo:3.6
  variables:
    MYSQL_DATABASE: "ci"
    MYSQL_ROOT_PASSWORD: "root"
  cache:
    key: "test"
    paths:
      - vendor/
  before_script:
    - export COMPOSER_ALLOW_SUPERUSER=1
    - /usr/local/bin/composer install --no-interaction --no-progress --no-scripts
    - ./artisan odm:clear-cache:metadata
    - ./artisan odm:generate:proxies
    - ./artisan odm:generate:hydrators
    - ./artisan clear-compiled
    - ./artisan cache:clear
    - ./artisan config:clear
    - ./artisan doctrine:clear:metadata:cache
    - ./artisan doctrine:generate:proxies
    - /usr/local/bin/composer dumpautoload
    - ./artisan doctrine:migrations:migrate
  script:
    - ./vendor/bin/phpunit

Used GitLab Runner version

11.11.1