rules:rule config contains unknown keys: exists

Summary

I've copied example using exists rule from this page and this rule not working.

Steps to reproduce

Try to lint this ci configuration:

job:
  script: docker build -t my-image:$CI_COMMIT_REF_SLUG .
  rules:
    - exists:
      - Dockerfile

What is the current bug behavior?

Status: syntax is incorrect 

Error: jobs:job:rules:rule config contains unknown keys: exists