Implement image_digest_tag

[jesse-r-s-hines]

rules_gitops pushes the images by digest without a tag, and has no options to set tags on the pushed images. Many container registries will eventually garbage collect images pushed without a tag which can break our deployment.

adobe/rule_gitops had an option image_digest_tag to avoid this, which just tagged all images with the short digest. fasterci/rule_gitops has stubs for that option as well but it isn't implemented. Can the image_digest_tag option be implemented as it was in adobe/rules_gitops?

Additionally it would be nice if there was some remote_tags option so you could set "latest" tags etc.

[jesse-r-s-hines]

I've submitted a proof-of-concept PR to implement this in #60 which we are using as a patch for the moment in our codebase. Though I'm not very familiar with creating bazel rules like this so may not have done it in the best way 😂 .