Add logging for update fails

[amouat]

At the moment it seems Frizbee will just fail silently if it can't get a digest e.g. because of auth or network issues. I think it would be worth logging this to stderr.

For example:

FROM redis:invalid_image_tag

is clearly an image reference, but will just be ignored.

I guess this is the place in the code?

https://github.com/stacklok/frizbee/blob/main/pkg/replacer/replacer.go#L303

[rdimitrov]

hey, @amouat 👋 Thanks for reaching out!

You're right, logging that message might be handy 👍 Feel free to open a PR in case you already have one 😃