disk usage

[ermalbaj]

Hi,
I would like to add some other features to monitor with docker_exporter like:

• Disk usage per container of the volumes
• Authentication of the docker_exporter to the Host like to use a user which will be in the docker group with limited permission or to run behind a nginx proxy with user/pass authentication. (i dont know any good scenario but if you have a better approach will be great to add as a feature for this)

Very good work. Thanks.