false is not a valid variable
Summary
According to the documentation https://docs.gitlab.com/ee/ci/yaml/#git-checkout you should set
variables:
GIT_STRATEGY: clone
GIT_CHECKOUT: false
But Gitlab then detects an invalid YAML syntax because. The YAML-Lint says: "variables config should be a hash of key value pairs"
variables:
GIT_STRATEGY: clone
GIT_CHECKOUT: "false"
is working.
Either fix documentation or change YAML validation.