Add Dependabot auto-approve workflow

Signed-off-by: Tiger Kaovilai <tkaovila@redhat.com>

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

@@ -0,0 +1,21 @@
+name: "Dependabot Auto Approve"
+# pull_request_target means that this will run on pull requests, but in the context of the base repo.
+# This should mean PRs from dependabot are supported.
+on:
+  pull_request_target:
+    types: [opened, reopened, synchronize, ready_for_review]
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  auto-approve:
+    if: github.repository == 'velero-io/velero' && github.event.pull_request.user.login == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Auto approve Dependabot PR
+        run: gh pr review --approve "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}