dockerfiles/rinkeby: added named volume for pg container

dockerfiles/rinkeby/docker-compose.yml

@@ -33,6 +33,8 @@ services:
       POSTGRES_USER: postgres
       POSTGRES_DB: vulcanizedb
       POSTGRES_PASSWORD: postgres
+    volumes:
+      - "vulcanizedb_db_data:/var/lib/postgresql/data"
     networks:
       vulcanizedb_net:
     healthcheck:
@@ -54,6 +56,7 @@ services:
 
 volumes:
   vulcanizedb_geth_data:
+  vulcanizedb_db_data:
 
 networks:
   vulcanizedb_net: