dockerfile ref: use custom ref to fix links issue

[crazy-max]

Changes merged in moby/buildkit#3220 cause htmlproofer errors: https://github.com/docker/docs/actions/runs/3319903637/jobs/5485635979#step:5:57

- ./_site/engine/reference/builder/index.html
  *  internally linking to /build/buildkit/, which does not exist (line 269)
     <a href="/build/buildkit/">BuildKit</a>
  *  internally linking to /build/buildkit/, which does not exist (line 2478)
     <a href="/build/buildkit/">BuildKit</a>
  *  internally linking to /build/buildkit/dockerfile-frontend/, which does not exist (line 272)
     <a href="/build/buildkit/dockerfile-frontend/">Custom Dockerfile syntax</a>
  *  linking to internal hash #getting-started that does not exist (line 3000)
     <a href="/build/buildkit/#getting-started">BuildKit</a>
- ./_site/engine/reference/commandline/build/index.html
  *  linking to internal hash #buildkit that does not exist (line 849)
     <a href="/engine/reference/builder/#buildkit">BuildKit backend</a>
  *  linking to internal hash #buildkit that does not exist (line 1114)
     <a href="/engine/reference/builder/#buildkit">enable BuildKit</a>
  *  linking to internal hash #buildkit that does not exist (line 1156)
     <a href="/engine/reference/builder/#buildkit">enable BuildKit</a>

While waiting for #15958 and docker/cli#3832 to be merged, use a custom ref for the Dockerfile reference before these changes.

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

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