Apply suggestion to doc/ci/docker/using_docker_images.md

doc/ci/docker/using_docker_images.md

@@ -235,12 +235,6 @@ test:2.7:
   - bundle exec rake spec
 ```
 
-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@sha256:a1a2ed1092c70ae6a7faf79863ccd5fe42ef696be49cb8e4f7c9dcf8a7e8b259
-```
-
 Or you can pass some [extended configuration options](#extended-docker-configuration-options)
 for `image` and `services`: