docs: cache_dir needs to appear in docker's volumes
docs: cache_dir needs to appear in docker's volumes

ayufan/gitlab-ci-multi-runner,hu19891110/gitlab-ci-multi-runner,ayufan/gitlab-ci-multi-runner,soiff/gitlab-ci-multi-runner,ayufan/gitlab-ci-multi-runner,hu19891110/gitlab-ci-multi-runner,hu19891110/gitlab-ci-multi-runner,soiff/gitlab-ci-multi-runner,soiff/gitlab-ci-multi-runner