Setup a docker container that holds a webdav instance to test files_exte...

...rnal

cc @DeepDiver1975 @PVince81 

Let's wait for an exploding Jenkins :)

Used docker container: https://registry.hub.docker.com/u/morrisjobke/webdav/ Source: https://github.com/MorrisJobke/owncloud-docker/tree/master/webdav
we should inject the config to be used via some env variable - this sedding feels unreliable