Restore docker buildx build now that runner has buildx plugin

Runner v2.5.1 includes docker-buildx-plugin, so we can switch back
to BuildKit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.forgejo/actions/build-push-image/action.yaml

@@ -30,7 +30,7 @@ runs:
       shell: bash
       run: |
         echo "Building ${{ inputs.image_name }}:${{ inputs.version }}"
-        docker build \
+        docker buildx build \
           --tag ${{ inputs.image_name }}:${{ inputs.version }} \
           --file ${{ inputs.context }}/${{ inputs.dockerfile }} \
           ${{ inputs.context }}