I am going to add curl to the docker images. It should be part of the nightly release(dgraph/dgraph:master) and also the next release. Once you have curl, you can initiate an export from any of the nodes in the cluster(from within a container running the dgraph server).
docker exec -it <container_id> bash
curl localhost:<dgraph_http_port>/admin/export
We have an open issue to add IP range whitelist from which exports can be initiated. We’ll also later add support for doing backups to cloud providers.