Adds docker.io support
Adds docker.io support

This (small) PR adds the docker-io package from Ubuntu Xenial as supported docker version

fixes #2775
jimjag/dokku,alexquick/dokku,elia/dokku,claytonbrown/dokku,alexquick/dokku,elia/dokku,dokku/dokku,jimjag/dokku,progrium/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,claytonbrown/dokku,elia/dokku,claytonbrown/dokku,elia/dokku,progrium/dokku,progrium/dokku,alexquick/dokku,elia/dokku,progrium/dokku,jimjag/dokku,jimjag/dokku,progrium/dokku,alexquick/dokku,jimjag/dokku,jimjag/dokku,elia/dokku,claytonbrown/dokku,alexquick/dokku,dokku/dokku,jimjag/dokku,alexquick/dokku,dokku/dokku,elia/dokku,dokku/dokku,claytonbrown/dokku,jimjag/dokku,dokku/dokku,dokku/dokku,alexquick/dokku,claytonbrown/dokku,alexquick/dokku,dokku/dokku,progrium/dokku,elia/dokku