whats the default docker machine image when registering a gitlab runner code example

Example 1: register runner gitlab

sudo gitlab-runner register
Please enter the gitlab-ci coordinator URL (e.g. https://gitlab.com ) e.g https://gitlab.com
Please enter the gitlab-ci token for this runner
Please enter the gitlab-ci description for this runner
Please enter the gitlab-ci tags for this runner (comma separated):my-tag,another-tag
Please enter the executor: ssh, docker+machine, docker-ssh+machine, kubernetes, docker, parallels, virtualbox, docker-ssh, shell:

Example 2: gitlab docker runner

docker run --rm -t -i gitlab/gitlab-runner --help

NAME:
   gitlab-runner - a GitLab Runner

USAGE:
   gitlab-runner <global options> command <command options> <arguments...>

VERSION:
   10.7.0 (7c273476)

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