Added info about private submodules

public-site/docs/guides/git-submodules/index.md

@@ -10,6 +10,10 @@ title: Git submodules
 
 As of December 2022, Radix only supports a single SSH key for authenticating to remote git repositories. This means that a Radix repository's submodules must be either public or accessible using the same deploy key as the main repository. Because GitHub globally prohibits reuse of a single deploy key across multiple repositories, it's not possible to use submodules which point to private GitHub repositories.
 
+:::info Note
+There is an option to update submodule within the pipeline job. Please look [at the example](/guides/git-submodules/update-submodule-in-pipeline-job.md).
+:::
+
 ## Sample application with git submodule
 
 [This GitHub repository](https://github.com/equinor/radix-app-with-submodule-example) contains a Radix application that has a single component, `redis`. This component is built from the source code inside the `redis` directory, and this directory is in turn a git submodule which points to a git repository at https://github.com/equinor/radix-submodule-example. Take note that the remote URL of the submodule is HTTPS and not SSH; unauthenticated clone via SSH, even for public repositories, is not supported.

public-site/docs/guides/git-submodules/update-submodule-in-pipeline-job.md

@@ -0,0 +1,110 @@
+---
+title: Update Git submodules in a pipeline job
+---
+
+# Update Git submodules in a pipeline job
+
+[Submodules](/guides/git-submodules/index.md) are mapped to a Git repository within a folder. This is an example of a Radix app with a submodule, located in the external private or internal Git repository.
+
+## Radix application repository structure
+```sh
+/
+├── app
+├── .gitmodules 
+├── Dockerfile
+└── radixconfig.yaml
+```
+* `app` - a "virtual" folder, referenced to a submodule
+* `.gitmodules` - a file, describing the reference to a submodule:
+
+    ```
+    [submodule "app"]
+        path = app
+        url = [email protected]:organisation-name/submodule-repository-name.git
+    ```
+## Submodule repository structure
+```sh
+/
+├── server.js
+```
+Repository content will be cloned to the Radix application cloned repository within the folder, specified in the `.gitmodules` property `path` (in this example in the folder `app`).
+## Radix config file
+radixconfig.yaml
+```yaml
+apiVersion: radix.equinor.com/v1
+kind: RadixApplication
+metadata:
+  name: your-radix-app-name
+spec:
+  build:
+    useBuildKit: true
+    secrets:
+      - SSH_KEY
+  environments:
+    - name: dev
+      build:
+        from: main
+  components:
+    - name: server
+      ports:
+        - name: http
+          port: 8080
+      publicPort: http
+```
+This Radix application has a build secret `SSH_KEY` and it uses build-kit to be built. 
+## Dockerfile
+```Dockerfile
+FROM docker.io/alpine AS builder
+
+RUN apk update && apk add git openssh-client
+RUN mkdir -p /root/.ssh && ssh-keyscan github.com > /root/.ssh/known_hosts
+
+WORKDIR /app
+COPY . .
+
+WORKDIR /root/.ssh
+RUN --mount=type=secret,id=SSH_KEY,dst=id_rsa \
+cd /app && git submodule update --init --remote --merge --recursive --verbose
+
+FROM docker.io/node:alpine
+
+WORKDIR /app
+COPY --from=builder /app .
+
+WORKDIR /app/app
+USER 1001
+EXPOSE 8080
+CMD ["node", "server.js"]
+```
+The build secret `SSH_KEY`, specified in the `radixconfig.yaml` should contain a private key (in PEM format), which will be mounted within the file `/root/.ssh/id_rsa` (which is used by default by Git CLI).
+This Dockerfile has two stages - the first (with an alias `builder`) is to update submodules, the second with a runtime to run the code.  
+If an option `--remote` is not specified - submodule will be cloned with a version referenced in the current commit of the Radix application repository, not the latest version of the submodule repository.
+
+:::info Hint
+The default file name with a private key can be changed with one of following options:
+* `env GIT_SSH_COMMAND='ssh -i /path/to/your/private_key' submodule update --init --recursive`
+* `git -c core.sshCommand="ssh -i /path/to/your/private_key" submodule update --init --recursive`
+:::
+## Prepare keys
+* Generate private and public keys. The key format need to be PEM, do not set passphrase (hit the Enter on the request "Enter passphrase" and "Enter same passphrase again"):
+    ```shell
+    ssh-keygen -t rsa -b 4096 -m PEM -f private-key-file
+    ```
+* Get generated keys with commands `cat` (Linux, MacOS, Windows PowerShell), `type` (Windows Terminal) or with an editor:
+  * the private key will be copy-pasted to the SSH_KEY build secret on the next step
+    ```shell
+    cat private-key-file
+    ```
+  * the public key need to be copy-pasted to a new deploy-key in the submodule's GitHub repository: `Repository/Settings/Deploy keys/Add deploy key`. `Allow write access` is not needed. 
+    ```shell
+    cat private-key-file.pem`
+    ```
+## Register and deploy Radix application
+* Register a Radix application in the Radix cluster
+* Create a first pipeline job `build-deploy`
+* This job will fail due to a build secret `SSH_KEY` is not populated. This can be fixed within the Radix console, "Configuration" page of the application, the section "Build secrets" - click on the secret name `SSH_KEY` and copy-paste the private key from the previous step
+* Create a new `build-deploy` pipeline job - when it is completed. Navigate to the job step `Building server-dev component`, ensure that log does not have an error on the step:
+  ```shell
+  [1/2] STEP 7/7: RUN --mount=type=secret,id=SSH_KEY,dst=id_rsa cd /app && git submodule update --init --remote --merge --recursive --verbose
+  ```
+* The application should be up and running