Merge pull request #3005 from flatcar/fix-cant-build-custom-image-on-fork

This pull request updates the README.md to include instructions for working with forks of the scripts repository. The new section explains how to fetch upstream tags to ensure proper version detection when using the SDK.

Documentation updates:

README.md: Added a new section titled "Working with forks" to guide users on fetching upstream tags when working with a fork of the scripts repository. This ensures compatibility with the SDK's version detection mechanism using git describe --tags.
Fixes: flatcar/Flatcar#1705

Author: John15321

README.md

@@ -102,6 +102,20 @@ To clone the scripts repo and pick a version:
   * list releases (e.g. all Alpha releases): `git tag -l alpha-*`
   * check out the release version, e.g. `3033.0.0`: `git checkout 3033.0.0`
 
+### Working with forks
+
+When using GitHub's "fork" feature, please **make sure to fork all branches**, not just `main`. Forking only `main` is the default on GitHub.
+
+The SDK container wrapper script `run_sdk_container` requires release tags in our release branches and fails to start if no release branch is present (see e.g. https://github.com/flatcar/Flatcar/issues/1705).
+If you have forked manually, please make sure to include all tags. You can retrofit upstream tags to a fork by using e.g.:
+
+```bash
+git remote add upstream https://github.com/flatcar/scripts.git
+git fetch --tags upstream
+```
+
+This is necessary because the SDK uses `git describe --tags` to determine the current version, and forks don't include the original repository's tags by default.
+
 To use the SDK container:
 * Fetch image and start the SDK container: `./run_sdk_container -t`
   This will fetch the container image of the "scripts" repo's release version you checked out.