cicleciでWarning: Git is not installed in the image. Falling back to CircleCI's native git client but this is still an experiment feature. We highly recommend using an image that has official Git installed. worktree is not clean

circleci 2.0でcheckoutするとタイトルみたいなエラーが出るんだよね。使ってるdocker imageにgitが入っていないので、gitを入れましょ。

apt-get install -y git