Add blank line, caught by CI.

doc/ci/docker/using_docker_images.md

@@ -230,6 +230,7 @@ test:2.7:
 ```
 
 Pinning images by digest (more detail in Docker's [Images: Tagging vs Digests](https://success.docker.com/article/images-tagging-vs-digests)) is accomplished with syntax like:
+
 ```yaml
 image: ruby:2.6@sha256:a1a2ed1092c70ae6a7faf79863ccd5fe42ef696be49cb8e4f7c9dcf8a7e8b259
 ```