Suggested use of MYSQL_ALLOW_EMPTY_PASSWORD in .gitlab-ci.yml invalid

Suggested use of variables in .gitlab-ci.yml at http://doc.gitlab.com/ci/docker/using_docker_images.html#configuring-services:

variables:
  MYSQL_ALLOW_EMPTY_PASSWORD: yes

...throws linting error:

variables should be a map of key-valued strings

This does work:

variables:
  MYSQL_ALLOW_EMPTY_PASSWORD: "yes"

... but so this does this:

variables:
  MYSQL_ALLOW_EMPTY_PASSWORD: "no"

• Documentation should be adjusted for current situation
• Parsing of variables should be changed (allow boolean or check the actual value of the string)