docs: HOWTO rebuild labs without commmits

Signed-off-by: Jean-Luc Davern <jdavern@linuxfoundation.org>

Author: jdavv

docs/HOWTOs/trigger-a-lab-rebuild-to-consume-upstream-changes.md

@@ -0,0 +1,51 @@
+# HOW TO Trigger a lab build to consume upstream changes
+
+This guide explains how to trigger a rebuild of an existing lab environment to incorporate upstream changes without modifying the lab content.
+
+## When to Use This Guide
+
+Use this guide when you need to:
+
+- Incorporate platform-level fixes, enhancements, or new features from the lab-building-service to an existing lab
+
+## Prerequisites
+
+Before you begin, ensure you:
+
+- Have access to the `lftraining` GitHub Organization
+- Are a member of the `lab-authors` team
+
+## Triggering a Lab Rebuild Using the Workflow
+
+1. Open the lab repository in your browser
+2. Navigate to the **Actions** tab in the repository
+3. Locate and select the `trigger lab build` workflow
+4. Click **Run workflow**
+5. In the dropdown menu for `Use workflow from`, select the branch containing your lab environment
+    - This is typically the branch where your lab content is defined (like `feature/implement-LFS2580004`)
+    - If rebuilding from main, select `main`
+6. Fill in the workflow parameters
+
+
+### Workflow Parameters
+
+- **Compute Environment Names** (required)
+    - Enter a comma-separated list of compute environment names you want to rebuild
+    - Example: `LFS2580004,LFS2580005`
+
+!!!important "Branch Selection"
+    The branch selection is part of GitHub's interface, not a workflow parameter, but still shows up in the same interface. Make sure to select the correct branch while entering the workflow parameters.
+
+## What Happens Next
+
+After triggering the workflow:
+
+1. The system identifies the specified compute environments
+2. Each environment is sent to the lab-building-service for rebuilding
+3. The labs are rebuilt with the latest platform updates
+4. No changes are made to your lab content or configuration
+5. A comment on the PR is added with the new lab version and a link to staging.
+
+!!!note "Build Time"
+    Rebuilding a lab can take 20-30 minutes depending on the complexity of the environment.
+    You can monitor progress in the Actions tab of the repository.