Gitea backup creates empty dump folder

when I follow the instructions on how to create a Gitea backup with docker I get an empty folder.

Commands that i ran:

winpty docker exec -u git -it $(docker ps -qf "name=gitea_server_1") bash

/app/gitea/gitea dump -c /data/gitea/conf/app.ini

after the last command it says:

2019/06/18 12:34:43 ...dules/setting/git.go:83:newGit() [I] Git Version: 2.20.1, Wire Protocol Version 2 Enabled

When I look into the folder tmp inside the container I see a folder named like gitea-dump-845633816, but the folder it is empty.

docker cp $(docker ps -qf "name=gitea_server_1"):/tmp/gitea-dump-845633816 

When I copy the file from the container only an empty folder with the same name is created.