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