Setup docker

Author: ldionne

.github/workflows/libcxx-build-and-test.yaml

@@ -293,6 +293,9 @@ jobs:
     # needs: [ stage2 ] # TODO: temporarily always trigger
     runs-on: llvm-premerge-libcxx-next-runners
     steps:
+      - name: Set up Docker
+        uses: docker/setup-docker-action@3fb92d6d9c634363128c8cce4bc3b2826526370a
+
       - name: Trigger Buildkite pipeline
         uses: buildkite/trigger-pipeline-action@f0dcd5ae74452a8d17d473227c9d212dcc1eb8f9 # v2.4.0
         with: