Ubuntu 18.04's docker packages predate compose v2. The plugin is a
single static binary that runs on any distro with docker CLI >= 18.09,
so the script downloads it into /usr/local/lib/docker/cli-plugins
(system-wide — sudo docker compose sees it too) when docker compose is
unavailable, with the same offline pattern as the python tarball: a
pre-downloaded binary next to install.sh wins. If the CLI is too old
to support plugins at all, the error asks for docker --version.