Use Docker volume for mysql data storage

.gitignore

@@ -2,7 +2,6 @@
 /public/storage
 /vendor
 /.idea
-/firefly-db-storage
 Homestead.json
 Homestead.yaml
 .env

docker-compose.prod.yml

@@ -9,7 +9,7 @@ services:
       - MYSQL_PASSWORD=firefly_db_secret
       - MYSQL_RANDOM_ROOT_PASSWORD=yes
     volumes:
-      - ./firefly-db-storage:/var/lib/mysql
+      - firefly-db-storage:/var/lib/mysql
 
   firefly-app:
     image: firefly-iii
@@ -28,3 +28,6 @@ services:
       - firefly-db
     volumes:
       - ./storage:/var/www/firefly-iii/storage
+volumes:
+  firefly-db-storage:
+    driver: local