set up repository mirroring

Set up GitHub Action to automatically mirror changes to master branch
to GitLab and Codeberg repositories.

Checking host keys is disabled because of yesolutions/mirror-action#14,
but this shouldn't be a problem (the code is public anyway).

.github/workflows/sync-mirrors.yml

@@ -0,0 +1,41 @@
+name: Sync mirrors
+
+on:
+  push:
+    branches:
+      - sync-test
+
+jobs:
+  sync-gitlab:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Sync GitLab mirror
+        uses: yesolutions/[email protected]
+        with:
+          REMOTE: 'ssh://[email protected]/arcctgx/ARver.git'
+          GIT_SSH_PRIVATE_KEY: ${{ secrets.ARVER_SYNC_PRIVATE_SSH_KEY }}
+          GIT_SSH_NO_VERIFY_HOST: true
+          PUSH_ALL_REFS: false
+
+  sync-codeberg:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Sync Codeberg mirror
+        uses: yesolutions/[email protected]
+        with:
+          REMOTE: 'ssh://[email protected]/arcctgx/ARver.git'
+          GIT_SSH_PRIVATE_KEY: ${{ secrets.ARVER_SYNC_PRIVATE_SSH_KEY }}
+          GIT_SSH_NO_VERIFY_HOST: true
+          PUSH_ALL_REFS: false