dhi: add guide to use in Kubernetes (#23393)

## Description

dhi: add guide to use in Kubernetes. For now, this only covers image
pull secrets.

## Reviews

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- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

Author: LaurentGoderre

content/manuals/dhi/how-to/_index.md

@@ -20,6 +20,10 @@ params:
       description: Learn how to pull, run, and reference Docker Hardened Images in Dockerfiles, CI pipelines, and standard development workflows.
       icon: play_arrow
       link: /dhi/how-to/use/
+    - title: Use a Docker Hardened Image in Kubernetes
+      description: Learn how to use Docker Hardened Images in Kubernetes deployments.
+      icon: play_arrow
+      link: /dhi/how-to/k8s/
     - title: Manage Docker Hardened Images
       description: Learn how to manage your mirrored and customized Docker Hardened Images in your organization.
       icon: reorder

content/manuals/dhi/how-to/k8s.md

@@ -0,0 +1,79 @@
+---
+title: Use a Docker Hardened Image in Kubernetes
+linktitle: Use an image in Kubernetes
+description: Learn how to use Docker Hardened Images in Kubernetes deployments.
+keywords: use hardened image, kubernetes, k8s
+weight: 35
+---
+
+{{< summary-bar feature_name="Docker Hardened Images" >}}
+
+## Authentication
+
+To be able to use Docker Hardened Images in Kubernetes, you need to create a 
+Kubernetes secret for pulling images from your mirror or internal registry.
+
+> [!NOTE]
+>
+> You need to create this secret in each Kubernetes namespace that uses a DHI.
+
+Create a secret using a Personal Access Token (PAT). Ensure the token has at least
+read-only access to private repositories. For Docker Hub replace `<registry server>`
+with `docker.io`.
+
+```console
+$ kubectl create -n <kubernetes namespace> secret docker-registry <secret name> --docker-server=<registry server> \
+        --docker-username=<registry user> --docker-password=<access token> \
+        --docker-email=<registry email>
+```
+
+To tests the secrets use the following command:
+
+```console
+kubectl apply --wait -f - <<EOF
+apiVersion: v1
+kind: Pod
+metadata:
+  name: dhi-test
+  namespace: <kubernetes namespace>
+spec:
+  containers:
+  - name: test
+    image: <your-namespace>/dhi-bash:5
+    command: [ "sh", "-c", "echo 'Hello from DHI in Kubernetes!'" ]
+  imagePullSecrets:
+  - name: <secret name>
+EOF
+```
+
+Get the status of the pod by running:
+
+```console
+$ kubectl get -n <kubernetes namespace> pods/dhi-test
+```
+
+The command should return the following result:
+
+```console
+NAME       READY   STATUS      RESTARTS     AGE
+dhi-test   0/1     Completed   ...          ...
+```
+
+If instead, the result is the following, there might be an issue with your secret.
+
+```console
+NAME       READY   STATUS         RESTARTS   AGE
+dhi-test   0/1     ErrImagePull   0          ...
+```
+
+Verify the output of the pod by running, which should return `Hello from DHI in Kubernetes!`
+
+```console
+kubectl logs -n <kubernetes namespace> pods/dhi-test
+```
+
+After a successful test, the test pod can be deleted with the following command:
+
+```console
+$ kubectl delete -n <kubernetes namespace> pods/dhi-test
+```