Setup buildx only if needed (#1012)

.github/workflows/readme-release.yml

@@ -171,6 +171,7 @@ jobs:
           password: ${{ secrets.DOCKERHUB_TOKEN }}
       -
         name: Set up docker buildx
+        if: env.VERSIONTAG_THIS != ''
         uses: docker/setup-buildx-action@v2
         with:
           install: true