improve wording on default buildkit image

Co-authored-by: Justin Chadwell <github@jedevc.com>
Signed-off-by: David Karlsson <david.karlsson@docker.com>

docs/guides/drivers/docker-container.md

@@ -32,7 +32,7 @@ pass to `--driver-opt`:
 
 ## Usage
 
-When you run a build, Buildx pulls the specified `moby/buildkit` image from
+When you run a build, Buildx pulls the specified `image` (by default, [`moby/buildkit`](https://hub.docker.com/r/moby/buildkit))
 [Docker Hub](https://hub.docker.com/u/moby/buildkit). When the container has
 started, Buildx submits the build submitted to the containerized build server.