docs: Update Container-Scanning docs

Update requirements with link to supported distributions

doc/user/application_security/container_scanning/index.md

@@ -43,6 +43,7 @@ To enable container scanning in your pipeline, you need the following:
   or [`kubernetes`](https://docs.gitlab.com/runner/install/kubernetes.html) executor.
 - Docker `18.09.03` or higher installed on the same computer as the runner. If you're using the
   shared runners on GitLab.com, then this is already the case.
+- An image matching [Clair's list of supported distributions](https://quay.github.io/claircore/).
 - [Build and push](../../packages/container_registry/index.md#build-and-push-by-using-gitlab-cicd)
   your Docker image to your project's container registry. The name of the Docker image should use
   the following [predefined environment variables](../../../ci/variables/predefined_variables.md):