Add runbook for creating test Docker distribution

Author: jpraynaud

docs/runbook/README.md

@@ -17,3 +17,5 @@ This page gathers the available guides to operate a Mithril network.
 | **Fix terraform lock**                  | [terraform-lock](./terraform-lock/README.md)                           | Fix a terraform lock in CD workflows.                                     |
 | **Manage SSH access to infrastructure** | [ssh-access](./ssh-access/README.md)                                   | Manage SSH access on the VM of the infrastructure for a user.             |
 | **Upgrade VM of infrastructure**        | [upgrade-vm](./upgrade-vm/README.md)                                   | Upgrade the VM of the infrastructure of a Mithril network.                |
+| **Create test Docker distribution**     | [test-docker-distribution](./test-docker-distribution/README.md)       | Create a custom test Docker distribution.                                 |
+

docs/runbook/test-docker-distribution/README.md

@@ -0,0 +1,31 @@
+# Create a custom test Docker distribution.
+
+## Introduction
+
+From time to time, we may need to create a test Docker distribution with custom arguments (notably the version of the Cardano CLi that is used).
+This is a convenient way for testing early releases of the Cardano node. This is also commonly used to create a Docker distribution from an unmerged PR.
+
+## Run the associated 'Docker images test' GitHub Actions workflow
+
+Go to the page of the workflow with your browser: [Docker image test](https://github.com/input-output-hk/mithril/actions/workflows/test-docker.yml)
+
+Then, click on the **Run workflow** button:
+
+![Run workflow button](./img/run-workflow-button.png)
+
+Then fill the form to manually run the workflow:
+
+![Run workflow form](./img/run-workflow-form.png)
+
+> [!WARNING]  
+> - The SHA of the commit must have been run successfully in the CI workflow beforehand
+
+> [!CAUTION]
+> It is highly recommended to run with the **Dry run** option checked at first and make sure that the process works as expected
+
+
+The result should look like this in the GitHub Actions:
+
+![Run workflow result](./img/run-workflow-result.png)
+
+You should also be able to see the newly created Docker images in the [package registry](https://github.com/orgs/input-output-hk/packages?repo_name=mithril)