docker build uses Buildx and BuildKit by default since Docker Engine 23.0.

And Kamal is using buildx to create the container images too.

By default its driver is configured to use docker-container.

There are other ways to run Buildkit though, too.

However, the leftover container is a known issue and it might now be known whether it's used by other services too.

Personally I believe that it can be safely stopped afterwards by default if this is a Desktop environment or if it is running from an explicit Kamal call.