Fixed linting error 'variables should be a map of key-valued strings'

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