Docker Build Cloud is a hosted BuildKit service from Docker Inc. You register a cloud builder with docker buildx create --driver cloud <org>/<name>, and subsequent docker build commands ship their build context to Docker’s remote builders instead of executing locally. The image layers come back, or get pushed directly to a registry from the cloud side.
Two things make it useful in practice. First, native multi-architecture builds: the service runs native ARM64 and AMD64 builders, so docker build --platform linux/amd64,linux/arm64 doesn’t fall back to QEMU emulation (which can be 10x slower for compile-heavy languages). Second, shared cache: the BuildKit cache lives on the remote builder and is shared across your team and CI jobs, so the layer your coworker built this morning is a cache hit when you build locally this afternoon. That eliminates the usual “CI cache is cold because each runner starts fresh” problem.
Docker Build Cloud is a commercial add-on to Docker paid plans. Alternatives are running your own remote BuildKit instance (the open-source piece does all of this, you just manage the machines), Depot.dev (a popular third-party remote build service that predates Docker’s offering), Namespace, Blacksmith, or the native multi-arch runners some CI vendors now provide. Under the hood it’s all the same BuildKit; the question is who runs the servers and cache.