Mark Docker as third-party software

As per the content guide, Docker is (essential) third party software.
Alternatives exist but Docker is certainly the most common tool used for
this kind of task.

Anyway, mark that it's third party.

content/en/docs/tasks/configure-pod-container/pull-image-private-registry.md

@@ -10,6 +10,7 @@ This page shows how to create a Pod that uses a
 {{< glossary_tooltip text="Secret" term_id="secret" >}} to pull an image from a
 private container image registry or repository.
 
+{{% thirdparty-content single="true" %}}
 
 ## {{% heading "prerequisites" %}}