`supports_compose_buildkit()` returned `!self.is_podman()`, assuming
every Docker install has buildx. On setups like Colima where the buildx
CLI plugin isn't installed, this causes builds to fail with "classic
builder doesn't support additional contexts" since `DOCKER_BUILDKIT=1`
alone isn't enough without the plugin.

Now probes for `docker buildx version` at construction time and caches
the result. When buildx isn't found, the existing scratch-image fallback
path (same one Podman uses) kicks in instead.

Closes #53890

Release Notes:

- Fixed dev container builds failing on Docker installations without the
buildx plugin.

---------

Co-authored-by: krisswee <krisswee@users.noreply.github.com>
Co-authored-by: KyleBarton <kjb@initialcapacity.io>