Feature/get images digests and repositories

[eesprit]

This Pull Request adds a new label on the podman_image_info metric : the sha256 Digest of the image

This is a more secure way to distinguish image when the tag might be overwritten.

With the digest, it is possible to identify the image in a registry even if the tag has been reused on a new image.

[navidys]

Hi @eesprit

Thanks for the PR.

DCO check failed, I would appreciate it if you can sign your commits and also update README.md metrics output examples.

[eesprit]

Hi @navidys

Thanks for four feedback.

I updated the README with some examples, and added the missing signoff on commits.

[navidys]

LGTM