added a volume for postgres data

docker-compose.yml

@@ -9,6 +9,8 @@ services:
              restart: always
              environment:
                POSTGRES_PASSWORD: mysecretpassword
+             volumes: 
+                - pgdata:/var/lib/postgresql/data
         nginx:
              build:
                 context: .
@@ -41,3 +43,5 @@ services:
              restart: always
              ports:
                - "8080:8080"
+volumes:
+        pgdata: