By default, nginx refuses to start if the upstream host is not found.
Since the webdav upstream server is optional here, we want nginx to
start even if the webdav upstream server is missing. The trick here is
to put the upstream url in a variable first so that nginx will not check
whether the upstream host exists on start and start anyway.
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.
