build: persist dev DB data (#283)

Persist the development DB in a Docker volume, so that it isn't lost when you shut down a local container.

docker-compose.dev.yml

@@ -14,6 +14,8 @@ services:
       POSTGRES_USER: postgres
     ports:
       - "5432:5432"
+    volumes:
+      - dev-data:/var/lib/postgresql/data
 
   gutenberg:
     image: gutenberg
@@ -34,4 +36,6 @@ services:
       db:
         condition: service_started
     environment:
-      DATABASE_URL: postgresql://postgres:super-secret-password@db:5432
+      DATABASE_URL: postgresql://postgres:super-secret-password@db:5432
+volumes:
+  dev-data: