ci: refactor docker runner script

ci: refactor docker runner script

We will support alpine check in docker later in this series.

While we're at it, tell people to run as root in podman,
if podman is used as drop-in replacement for docker,
because podman will map host-user to container's root,
therefore, mapping their permission.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>