ci: add redis service

.github/workflows/ci.yml

···
 jobs:
   tests:
     runs-on: ubuntu-latest
+
+    services:
+      redis:
+        image: redis:7-alpine
+        ports:
+          - 6379/tcp
+        options: >-
+          --health-cmd "redis-cli ping || exit 1"
+          --health-interval 5s
+          --health-timeout 3s
+          --health-retries 10
+
     steps:
       - uses: actions/checkout@v2
       - name: Install dependencies