Add dependabot for GitHub Actions dependencies

.github/dependabot.yml

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+# To get started with Dependabot version updates, you'll need to specify which
+# package ecosystems to update and where the package manifests are located.
+# Please see the documentation for more information:
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+# https://containers.dev/guide/dependabot
+
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/test-dependabot.yml

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+name: Check dependabot update
+
+on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - .github/workflows/*.yml
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  test-feature:
+    uses: ./.github/workflows/test-feature.yml
+  dependabot-auto-approve:
+    needs:
+      - test-feature
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - name: Dependabot metadata
+        id: dependabot-metadata
+        uses: dependabot/fetch-metadata@v1.6.0
+        with:
+          github-token: '${{ secrets.GITHUB_TOKEN }}'
+      - name: Enable auto-merge for Dependabot PRs
+        run: gh pr merge --auto --merge "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}