contributing: require DCO

DCO requires contributors to certify that they have the right to
submit their patches under the project's open-source license.
This establishes a clear chain of responsibility for the copyright
status of the code.

The rise of generative AI use is the main driver for this change:
the copyright situation is still unclear and there is no reliable
way to tell apart contributions made (in part) with generative AI.
Note, DCO had some legal upsides even before generative AIs
existed.

A new CI step has been added to check that all commits contain at
least one Signed-off-by trailer.

.builds/alpine.yml

@@ -58,3 +58,14 @@ tasks:
         -o public \
         "$include_dir/wlr/"
       zip -r ~/public.zip public/
+  - check-dco: |
+      set +x
+      cd wlroots
+      [ "${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-}" = "" ] && exit
+      git rev-list "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME".. | while read -r rev; do
+        sob=$(git log -1 --pretty='%(trailers:key=Signed-off-by,separator= )')
+        if [ "$sob" = "" ]; then
+          echo "Commit $rev is missing a Signed-off-by trailer"
+          exit 1
+        fi
+      done

CONTRIBUTING.md

@@ -101,6 +101,17 @@ message as well.
 
 See [How to Write a Git Commit Message] for more details.
 
+## Developer Certificate of Origin
+
+All commits must include a `Signed-off-by` trailer with your preferred name and
+email address which indicates that you agree to the [Developer Certificate of
+Origin]. If part of the patch has been authored by someone else, you should
+also gather `Signed-off-by` trailers from them in addition to your own trailer.
+These trailers certify that the commit can be submitted under the project's
+open-source license.
+
+The trailer can be added with `git commit -s`.
+
 ## Code Review
 
 When your changes are submitted for review, one or more core committers will
@@ -473,6 +484,7 @@ static void subsurface_handle_surface_destroy(struct wl_listener *listener,
 [freedesktop wiki]: https://gitlab.freedesktop.org/freedesktop/freedesktop/-/wikis/home
 [linear, "recipe" style]: https://www.bitsnbites.eu/git-history-work-log-vs-recipe/
 [git-rebase.io]: https://git-rebase.io/
+[Developer Certificate of Origin]: https://developercertificate.org/
 [reference issues]: https://docs.gitlab.com/ee/user/project/issues/managing_issues.html#closing-issues-automatically
 [Code of Conduct]: https://www.freedesktop.org/wiki/CodeOfConduct/
 [How to Write a Git Commit Message]: https://chris.beams.io/posts/git-commit/