Add images digests to the outputs fields

[sxd]

Description

Hi all!

Currently there's only one output field metadata, while this output is useful, it will be nice to have something like digest in docker-push-action where you can have all the images with the digest in one output.

Why this is useful?

When signing the images using https://github.com/sigstore/cosign the sha256 is the recommend process to sign an image, and this could be passed to directly sign the images coming from the output of the bake action and everything with just two actions instead of passing parsing the output of the metadata.

Regards,