Enable dependabot (#1)

Signed-off-by: Taiki Ono <[email protected]>

.github/dependabot.yml

@@ -0,0 +1,25 @@
+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      # Avoid default day of week because of potential rush.
+      day: "tuesday"
+      time: "12:00"
+      timezone: "Asia/Tokyo"
+    # To avoid lock file conflicts, group all updates together.
+    groups:
+      cargo-all:
+        patterns:
+          - "*"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+      time: "12:00"
+      timezone: "Asia/Tokyo"
+    groups:
+      github-actions-all:
+        patterns:
+          - "*"

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

@@ -0,0 +1,29 @@
+# https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions#enable-auto-merge-on-a-pull-request
+#
+# Our release process is not automated, so auto-merge risk can be considered low.
+name: Dependabot auto-merge
+on: pull_request
+
+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@5e5f99653a5b510e8555840e80cbf1514ad4af38 # v2.1.0
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+      - name: Enable auto-merge for Dependabot PRs
+        # github-actions dependencies are used immediately, so avoid auto-merging them.
+        if: |
+          steps.metadata.outputs.package-ecosystem == 'cargo' &&
+          (steps.metadata.outputs.update-type == 'version-update:semver-minor' || steps.metadata.outputs.update-type == 'version-update:semver-patch')
+        run: gh pr merge --auto --squash "${PR_URL}"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}