ci: drop the Docker gha cache that broke the image push

The Docker preview job now fails outright:

  #14 exporting to GitHub Actions Cache
  #14 ERROR: error writing layer blob: failed to reserve cache
  #13 exporting to image ... CANCELED
  ERROR: failed to build: failed to solve: error writing layer blob

The cache export is fatal to the build, so it cancelled the image push. My
optimization broke the job it was meant to speed up, and the npm preview for
PR #2328 published fine while its Docker image did not.

Reverting rather than fixing it. Making it work would need `actions: write`
on the one job that installs and executes the preview package, which is the
job SR-5 says to keep as unprivileged as possible, and this was the only
`type=gha` usage in the repo so there was no working precedent to copy. The
benefit was 60-90s of apt on a path that already waits on a human approval
measured in minutes to days, so it was buying almost nothing.

`ignore-error=true` would keep the build green but the export would keep
failing, leaving a dead directive and a stack trace in every log.

Author: fengmk2

.github/workflows/publish-preview-register.yml

@@ -454,10 +454,6 @@ jobs:
           tags: ${{ env.IMAGE }}:pr-${{ needs.authorize.outputs.pr }}
           build-args: |
             VP_PR_VERSION=${{ needs.authorize.outputs.pr }}
-          # The Dockerfile always comes from the default branch, so its apt layer
-          # is identical across previews. Only the final install layer varies.
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
           # Single-manifest image (no attestation index): simpler for consumers
           # and lets the comment job read the size via `docker manifest inspect`.
           provenance: false