Fix missing persistent volume for PostgreSQL data (#78)

Author: KOliver94

backend/docker-compose.yml

@@ -20,5 +20,10 @@ services:
       interval: 5s
       timeout: 3s
       retries: 5
+    volumes:
+      - pgdata:/var/lib/postgresql
     ports:
       - "127.0.0.1:5432:5432"
+
+volumes:
+  pgdata:

docker-compose.yaml

@@ -26,5 +26,10 @@ services:
       interval: 5s
       timeout: 3s
       retries: 5
+    volumes:
+      - pgdata:/var/lib/postgresql
     ports:
       - "127.0.0.1:5432:5432"
+
+volumes:
+  pgdata: