dockerfile ref: use custom ref to fix links issue (docker#15972)

Signed-off-by: CrazyMax <[email protected]>

Signed-off-by: CrazyMax <[email protected]>
Co-authored-by: CrazyMax <[email protected]>

Author: crazy-max

_config.yml

@@ -190,7 +190,7 @@ fetch-remote:
 
   - repo: "https://github.com/moby/buildkit"
     default_branch: "master"
-    ref: "master"
+    ref: "8bc51649bc7e712a8e2ff90412f48fdc35f602a9"
     paths:
       - dest: "engine/reference/builder.md"
         src: