Merge branch '3147-gitlab-ci-yml-variables-adjust-docs' into 'master'

Fixed suggestion in documentation resulting in linting error 'variables should be a map of key-valued strings'

Documentation now suggests a valid option with the intended result.

See #3147 

`"yes"`, opposed to `yes` is not casted to a boolean. `variables` only accepts key-valued strings.

Better solution would be to change the parsing of `variables` (allow boolean or check the actual value of the string).



See merge request !1660

doc/ci/docker/using_docker_images.md

@@ -90,7 +90,7 @@ you need to set MYSQL_ALLOW_EMPTY_PASSWORD.
     - mysql
     
     variables:
-      MYSQL_ALLOW_EMPTY_PASSWORD: yes
+      MYSQL_ALLOW_EMPTY_PASSWORD: "yes"
     ```
 
 For other possible configuration variables check the