website/lang/en/docs/_ci/gitlab.md

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Because GitLab CI uses docker in the background, you can specify an image with yarn pre-installed.

# .gitlab-ci.yml
image: node:9.4.0

If you're using a docker image that doesn't come with yarn pre-installed you can still install it after the container has loaded.

# .gitlab-ci.yml
image: does-not-have-yarn

before_script:
  # Install yarn as outlined in (https://yarnpkg.com/lang/en/docs/install/#alternatives-stable)
  - curl -o- -L https://yarnpkg.com/install.sh | bash
  # Make available in the current terminal
  - export PATH="$HOME/.yarn/bin:$HOME/.config/yarn/global/node_modules/.bin:$PATH"

In either case, it's good practice to cache your .yarn folder as well to speed up your builds.

# .gitlab-ci.yml
cache:
  paths:
    - .yarn

Here's an example .gitlab-ci.yml file using yarn to run a testing suite.
Just save this file to the root of your project and GitLab's CI will pick up the jobs.

# .gitlab-ci.yml
image: node:9.11.1

before_script:
  - yarn install --cache-folder .yarn

test:
  stage: test
  cache:
    paths:
    - node_modules/
    - .yarn