Add DinD example note for k8s executors

Using the DinD example that stores the built image in the container
registry may not be visible to the Kubernetes executor if the default
image pull policy defaults to IfNotPresent.

doc/ci/docker/using_docker_build.md

@@ -721,6 +721,13 @@ deploy:
     - master
 ```
 
+NOTE: **Note:**
+This example explicitly calls `docker pull`. If you prefer to implicitly pull the
+built image using `image:`, and use either the [Docker](https://docs.gitlab.com/runner/executors/docker.html)
+or [Kubernetes](https://docs.gitlab.com/runner/executors/kubernetes.html) executor,
+make sure that [`pull_policy`](https://docs.gitlab.com/runner/executors/docker.html#how-pull-policies-work)
+is set to `always`.
+
 [docker-in-docker]: https://blog.docker.com/2013/09/docker-can-now-run-within-docker/
 [docker-cap]: https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities
 [2fa]: ../../user/profile/account/two_factor_authentication.md