title: GitLab CI docker in docker can't create volume
body: <p>I'm using docker in docker to host my containers as they work through the pipeline. The container I create from my code is setup to have a volume to pass in a gcloud key to the container. This works perfectly on my local machine, but on the gitlab-runner it doesn't link correctly. </p>
  
  <p>From reading this appears to be because it links the host to my container, rather than the dind host to my container. </p>
  
  <p>How do I link the directory that is inside dind to my container?</p>
  
  <p>(Also ignore any minor issues with tagging and such, this ci file is very early in development)</p>
  
  <p>GitLab ci below</p>
  
  <pre><code>image: docker:latest
  services:
    - docker:dind
  
  
  variables:
    DOCKER_DRIVER: overlay2
    SPRING_PROFILES_ACTIVE: gitlab-ci
    CONTAINER_TEST_IMAGE: registry.gitlab.com/fdsa
    CONTAINER_RELEASE_IMAGE: registry.gitlab.com/asdf
  
  stages:
    - build_test_image
    - deploy
  
  .docker_login: &amp;docker_login | # This is an anchor
       docker login -u gitlab-ci-token -p $CI_JOB_TOKEN registry.gitlab.com
  
  build test image:
      stage: build_test_image
      script:
        - *docker_login
        - docker build -t $CONTAINER_TEST_IMAGE .
        - docker push $CONTAINER_TEST_IMAGE
  
  test run:
      stage: deploy
      script:
          - *docker_login
          - mkdir /key
          - echo $GCP_SVC_KEY &gt; /key/application_default_credentials.json
          # BROKEN LINE HERE
          - docker run --rm -v "/key:/.config/gcloud/" $CONTAINER_TEST_IMAGE
      tags:
          - docker
  </code></pre>