Add clarification about docker

doc/ci/quick_start/index.md

@@ -117,15 +117,22 @@ The pipeline starts when the commit is committed.
 
 #### `.gitlab-ci.yml` tips
 
-- If you want the runner to use a Docker image to run the jobs, edit the `.gitlab-ci.yml` file
-  to include your image name:
+- If you want the runner to [use a Docker container to run the jobs](../docker/using_docker_images.md),
+  edit the `.gitlab-ci.yml` file
+  to include an image name:
 
   ```yaml
   default:
     image: ruby:2.7.2
   ```
 
-  This command tells the runner to use a Ruby image from Docker Hub.
+  This command tells the runner to use a Ruby image from Docker Hub
+  and to run the jobs in a container that's generated from the image.
+
+  This process is different than
+  [building an application as a Docker container](../docker/using_docker_build.md).
+  Your application does not need to be built as a Docker container to
+  run CI/CD jobs in Docker containers.
 
 - To validate your `.gitlab-ci.yml` file, use the
   [CI Lint tool](../lint.md), which is available in every project.