title: GitLab: Mount /builds in build image as tmpfs
body: <p>In our GitLab CI environment we have a build server with lots of RAM but mechanical disks, running npm install takes a long time (I have added cache but it still needs to chew through existing packages so cache cannot solve all of this alone).</p>
  
  <p>I want to mount /builds in the builder docker image as tmpfs but I'm having a hard time figuring out where to put this configuration. Can I do that in the builder image itself or maybye in .gitlab-ci.yml for each project?</p>
  
  <p>Currently my gitlab-ci.yml looks like this:</p>
  
  <pre><code>image: docker:latest
  
  services:
    - docker:dind
  
  variables:
    DOCKER_DRIVER: overlay
  
  cache:
    key: node_modules-${CI_COMMIT_REF_SLUG}
    paths:
      - node_modules/
  
  stages:
    - test
  
  test:
    image: docker-builder-javascript
    stage: test
    before_script:
      - npm install
    script:
      - npm test
  </code></pre>