fix #755: Tests fail with new Docker version scheme
fix #755: Tests fail with new Docker version scheme

Docker introduced a [new version scheme](https://blog.docker.com/2017/03/docker-enterprise-edition/). As the result, the latest docker version is now `17.03.0-ce`.

thomasleveil/nginx-proxy,itsafire/nginx-proxy,thomasleveil/nginx-proxy,kamermans/nginx-proxy,pocket-playlab/nginx-proxy,krknopp/docker-nginx-proxy,jwilder/nginx-proxy,jwilder/nginx-proxy,kamermans/nginx-proxy,pocket-playlab/nginx-proxy,itsafire/nginx-proxy,krknopp/docker-nginx-proxy,krknopp/docker-nginx-proxy