DOCKER_BUILDKIT=1 in the build workflow only works if the host has
the buildx CLI plugin installed. Bare docker-ce sometimes ships
without it (Debian/Ubuntu separate packaging). Document the install
so future hosts onboarding to be build targets don't hit the same
'BuildKit is enabled but the buildx component is missing' error.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>