Verify Docker Buildx build actually uses the native node

How do I verify a Docker Buildx build actually uses the native node for a given platform?

I have a QEMU-base multi-platform Buildx builder called maven on x64 Linux. To that I appended a Mac Mini as native node for Arm builds.

$ docker buildx ls
NAME/NODE  DRIVER/ENDPOINT             STATUS   BUILDKIT PLATFORMS
builder    docker-container                              
  builder0 unix:///var/run/docker.sock inactive          
maven *    docker-container                              
  maven0   unix:///var/run/docker.sock running  v0.11.0  linux/amd64*, linux/amd64/v2, linux/arm64, ....
  mac-mini ssh://some-user@10.1.2.3    running  v0.11.5  linux/arm64*, linux/amd64, linux/amd64/v2, ...

Also, I verified the setup actually works by building a "Hello World" image.

$ docker buildx build --progress plain --builder maven --platform linux/arm64,linux/amd64 --tag my-hello-world .
...
#8 [linux/arm64 internal] load metadata for docker.io/library/debian:bullseye-slim
#8 DONE 1.0s

#6 [linux/amd64 internal] load metadata for docker.io/library/debian:bullseye-slim
#6 DONE 0.9s
...

The question is: did Docker actually use the native node for the Arm build or maybe still the local QEMU "node"? The output doesn't say.

The Buildx driver docker-container creates a moby/buildkit container on each node. The name of the container is buildx_buildkit_<node-name>. You can poke around that container to understand what it's doing.

Hence, either on the "local" host or on the remote Mac Mini do

• $ docker ps to verify what the container name is e.g. buildx_buildkit_mac-mini in my case
• $ docker exec -ti buildx_buildkit_mac-mini sh to get console access to that running container
• $ top inside the container to monitor processes

Inside those containers PID 1 is always buildkitd. Other process will be started if you start a Buildx build and Docker assigns work to this node. Examples below.

Idle

Busy