GitLab CI/CD: Run jobs only when files in a specific directory have changed

I would like to run specific jobs on the .gitlab-ci.yaml if and only if files within specific directories of the repository have changed. Is there a way to do this with gilab's ci/cd tooling or would it be easier just to run a custom build script?

Changes policy introduced in GitLab 11.4.

For example:

docker build:
  script: docker build -t my-image:$CI_COMMIT_REF_SLUG .
  only:
    changes:
      - Dockerfile
      - docker/scripts/*
      - dockerfiles/**/*
      - more_scripts/*.{rb,py,sh}

In the scenario above, if you are pushing multiple commits to GitLab to an existing branch, GitLab creates and triggers the docker build job, provided that one of the commits contains changes to either:

• The Dockerfile file.
• Any of the files inside docker/scripts/ directory.
• Any of the files and subdirectories inside the dockerfiles directory.
• Any of the files with rb, py, sh extensions inside the more_scripts directory.

You can read more in the documentation

this is now possible:

rules:
    - changes:
      - package.json
      - yarn.lock
    - exists: 
      - node_modules
      when: never
    - when: always

You can either check for file changes or check existence. Rules Documentation.

You can do something like this:

trigger_documentation:
  image: alpine:3.8
  stage: trigger_documentation
  script:
    - apk update && apk add git curl bash
    - |
      if git diff --name-only --diff-filter=ADMR @~..@ |grep ^doc/; then
            echo "triggering..."
      fi