Add redis cache to postgres testcase

.github/workflows/postgresql.yaml

@@ -27,6 +27,7 @@ jobs:
       INVENTREE_DEBUG: info
       INVENTREE_MEDIA_ROOT: ./media
       INVENTREE_STATIC_ROOT: ./static
+      INVENTREE_CACHE_HOST: localhost
 
     services:
       postgres:
@@ -37,6 +38,11 @@ jobs:
         ports:
           - 5432:5432
 
+      redis:
+        image: redis
+        ports:
+          - 6379:6379
+
     steps:
       - name: Checkout Code
         uses: actions/checkout@v2