added preinstall script description to 2.18

Author: ozRunAI

docs/admin/runai-setup/cluster-setup/cluster-prerequisites.md

@@ -384,20 +384,20 @@ To use the script [download](https://github.com/run-ai/preinstall-diagnostics/re
 
 
 === "SaaS"
-* On EKS deployments, please run aws configure prior to execution
+    * On EKS deployments, please run `aws configure` prior to execution
 
     ``` bash
-    chmod +x ./preinstall-diagnostics-darwin-arm64 && \
-  ./preinstall-diagnostics-darwin-arm64 \
+    chmod +x ./preinstall-diagnostics-<platform> && \
+    ./preinstall-diagnostics-<platform> \
       --domain ${TENANT_NAME}.run.ai \
       --cluster-domain ${CLUSTER_FQDN}
     ```
 
 === "Self-hosted"
 
-
     ``` bash
-        chmod +x ./preinstall-diagnostics-darwin-arm64 && ./preinstall-diagnostics-darwin-arm64 \
+    chmod +x ./preinstall-diagnostics-<platform> && \ 
+    ./preinstall-diagnostics-<platform> \
       --domain ${CONTROL_PLANE_FQDN} \
       --cluster-domain ${CLUSTER_FQDN} \
     #if the diagnostics image is hosted in a private registry
@@ -409,10 +409,26 @@ To use the script [download](https://github.com/run-ai/preinstall-diagnostics/re
 
     On an air-gapped deployment, the diagnostics image should be pulled, saved and manually pushed to the organization's registry.
 
-    The binary should be run with --image parameter to modify the diagnostics image to be used:
-
+    The binary should be run with `--image` parameter to modify the diagnostics image to be used:
 
+    ``` bash
+    #Save the image locally
+    docker save --output preinstall-diagnostics.tar gcr.io/run-ai-lab/preinstall-diagnostics:${VERSION}
+    #Load the image to the organization's registry
+    docker load --input preinstall-diagnostics.tar
+    docker tag gcr.io/run-ai-lab/preinstall-diagnostics:${VERSION} ${CLIENT_IMAGE_AND_TAG} 
+    docker push ${CLIENT_IMAGE_AND_TAG}
+    ```
 
+    Finally, run the diagnostics tool:
+    ``` bash
+    chmod +x ./preinstall-diagnostics-darwin-arm64 && \
+    ./preinstall-diagnostics-darwin-arm64 \
+      --domain ${CONTROL_PLANE_FQDN} \
+      --cluster-domain ${CLUSTER_FQDN} \
+      --image-pull-secret ${IMAGE_PULL_SECRET_NAME} \
+      --image ${PRIVATE_REGISTRY_IMAGE_URL}    
+    ```
 
 
 If the script shows warnings or errors, locate the file `runai-preinstall-diagnostics.txt` in the current directory and send it to Run:ai technical support.