Add the example of a docker-compose for the database.

docker/docker-compose.yml

@@ -0,0 +1,14 @@
+version: '3'
+
+services:
+  db:
+    image: postgres:15-alpine
+    restart: always
+    volumes:
+      - ./data:/var/lib/postgresql/data
+    environment:
+      POSTGRES_DB: testdb
+      POSTGRES_USER: user
+      POSTGRES_PASSWORD: testpassword
+    ports:
+      - 5432:5432