Add parameter "--list" to list all tags for the same Image

Hi,

Is it possible to extend your great tool in that way that I can do the following command:
`./docker-retag --list "dcso:2.4.113-debian-dev"`
As Output I get than:
2.4.113-debian-dev
latest-dev

So the parameter "--list" should download the digest from the requested tag and compare it with all other available tags to find all tags which are from the same image. Without the need to download all images.
With the list I can then retag also these tags as result:
~~~~bash
for i in $(./docker-retag --list "dcso:2.4.113-debian-dev")
do
k="$(echo "$i"|sed 's,-dev$,,')" # without dev suffix
./docker-retag "$i" "$k"
done
~~~~