Show instance disk space

[JHSaunders]

It would be nice to see how much disk space is available on an instance. This is a little more complicated since an instance can have multiple disks, EBS volumes, or even use EFS. But I think we should be able to create a system that is both general and in the 95% case attractive.

[UnaKarlsen]

Having the Storage driver space, as reported by "docker info", would be a great start. If using devicemapper and your volume can grow into the storage, then querying LVM would be important too.

The output of df could be put into an array to be displayed later, though df isn't great when using btrfs.