Proper age for images

[efrecon]

Currently, the utility uses the image creation date, coming from the JSON response from docker image inspect to guess the age of an image. This is how old the image is, but not has little to do with when it was first put into action on the local system and started to be used by some container.

Instead, the age should be computed as to when the image was first put onto the local file system. Given an image SHA256 (or short) ID, e.g. sha256:a39fffbbe2d0346fa54587a1cf7273599cb519187ca24516d7e4e85acab72d5e or a39fffbbe2d0, the date can be found at the file with the same name as the SHA256, without the leading sha256: string, in the directory that contains the database of images, e.g. /var/lib/docker/image/aufs/imagedb/content/sha256. Where this content really is has with the local installation of docker. But docker info is able to return back the root directory of the storage driver, e.g. /var/lib/docker/aufs. However, this will vary depending on the configuration and the underlying filesystem.