ci: automatically merge Dependabot PRs (#25)

.github/workflows/auto-merge.yml

@@ -0,0 +1,23 @@
+name: Merge dependency updates
+
+on:
+    workflow_run:
+        types:
+            - completed
+        workflows:
+            - 'Docker build'
+            - 'Lint files'
+
+jobs:
+    merge-dependency-updates:
+        name: Merge dependency updates
+        runs-on: ubuntu-latest
+        steps:
+            - name: Merge dependency updates
+              if: ${{ github.event.workflow_run.conclusion == 'success' }}
+              uses: ridedott/merge-me-action@v2
+              with:
+                  # This must be used as GitHub Actions token does not support pushing
+                  # to protected branches.
+                  GITHUB_TOKEN: ${{ secrets.AUTO_MERGE_PR_TOKEN }}
+                  PRESET: DEPENDABOT_MINOR