Fix edgecase docker.io image reported as unused

[AiroPi]

Currently, to check if an image is used or not, the reference is checked.

This can cause edge-case issues with docker.io being masked by Docker.

To reproduce you can make this:

docker run docker.io/hello-world:latest

(Note the use of explicit docker.io)

The container will have docker.io/hello-world:latest as image, and the image reference will be hello-world:latest.

This cause the image to be reported as unused because it will test hello-world:latest against docker.io/hello-world:latest which don't match.

To address this issue, I change the comparison to use image id instead. It's my first time with rust btw.

(nb: I was not able to test for swarm so the image id is set as None for now)