Dependabot: configure monthly updates for gem dependencies

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+updates:
+  - directory: /
+    package-ecosystem: bundler
+    versioning-strategy: lockfile-only
+    allow:
+      - dependency-type: all
+    schedule:
+      interval: monthly
+    open-pull-requests-limit: 50
+    groups:
+      ruby:
+        patterns:
+          - '*'
+    commit-message:
+      prefix: Dependabot

.github/workflows/dependabot-auto-merge.yml

@@ -0,0 +1,26 @@
+name: Dependabot auto-merge
+
+on:
+  pull_request_target:
+    paths:
+      - Gemfile.lock
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v1
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+      - name: Enable auto-merge for Dependabot PRs
+        run: gh pr merge "$PR_URL" --auto --squash
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}