You can run this container in following way. You can then access the WebDAV instance at http://localhost:8888/. Internally the folder /var/webdav is used as WebDAV root.
docker run -d -e USERNAME=test -e PASSWORD=test -p 8888:80 exprime/webdav
For data persistance/access use a volume.
docker run -d -e USERNAME=test -e PASSWORD=test -p 8888:80 -v some/dir:/var/webdav exprime/webdav