Fix docker example

[danog]

The order of the base stages is wrong.

[jathak]

It's not clear to me why this change is necessary. Can you share the error message you get when running the example as it currently exists?

[danog]

[daniil@pi4 a]$ DOCKER_BUILDKIT=1 docker build .
[+] Building 0.5s (2/2) FINISHED            docker:default
 => [internal] load .dockerignore                     0.2s
 => => transferring context: 2B                       0.0s
 => [internal] load build definition from Dockerfile  0.2s
 => => transferring dockerfile: 445B                  0.0s
ERROR: failed to solve: circular dependency detected on stage: buf