Github action mirror to Gitlab

.github/workflows/mirror.yml

+name: Mirroring to Concordant Gitlab
+
+on: [push, delete]
+
+jobs:
+  to_gitlab:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - uses: pixta-dev/repository-mirroring-action@v1
+        with:
+          target_repo_url:
+            git@gitlab.inria.fr:concordant/software/c-sudoku.git
+          ssh_private_key:
+            ${{ secrets.GITLAB_SSH_PRIVATE_KEY }}
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