revert: remove registry cache backend, keep native Docker layer cache

--cache-to with docker driver doesn't support registry backend.
Host daemon already persists build layers via shared socket.
No changes needed - cache works automatically.

.gitea/workflows/pr.yml

@@ -60,19 +60,11 @@ jobs:
           echo "pr_number=$(echo $GITHUB_REF | grep -oP 'refs/pull/\K[0-9]+')" >> $GITHUB_OUTPUT
           echo "short_sha=$(echo $GITHUB_SHA | cut -c1-7)" >> $GITHUB_OUTPUT
 
-      - name: Login to Registry
-        run: |
-          docker login ${{ env.REGISTRY }} -u ${{ secrets.DOCKER_LOGIN }} -p ${{ secrets.DOCKER_PASSWORD }}
-
       - name: Build Docker image for PR
-        env:
-          DOCKER_BUILDKIT: 1
         run: |
           IMAGE_TAG="pr-${{ steps.info.outputs.pr_number }}-${{ steps.info.outputs.short_sha }}"
           docker build \
             --target runner \
-            --cache-from type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:cache \
-            --cache-to type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:cache,mode=max \
             --build-arg GIT_COMMIT=$GITHUB_SHA \
             -t ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${IMAGE_TAG} \
             .