I have not exported any data from dgraph yet, but you are saying that you can see the export data within the /dgraph folder in the container. According to your docker-compose file this folder is mapped to /tmp/data on your local machine. So have a look inside that folder, if I’m not mistaken your export data should be there.
You could map your /dgraph folder also to some other folder on your machine. I personally map it to my project folder:
volumes:
- ./data:/dgraph