This patch provides an optional way to integrate an external WebDAV server so that the super-productivity container can serve as a WebDAV server with base url http://localhost/webdav/ . It includes the following changes:
**Replace the default nginx config file**
Besides serving the web app, the new nginx config file also forwards all the requests with paths starting with "/webdav/" to a backend WebDAV server specified by the environment variable WEBDAV_BACKEND. Note that during forwarding, the path prefix "/webdav" will be removed.
**Use hacdias/webdav as default WebDAV backend server**
The docker-compose.yaml provides an example setup to use the docker image [hacdias/webdav](https://github.com/hacdias/webdav) as the WebDAV backend server. An example for the configuration of the WebDAV server is also provided in webdav.yaml.
