Specify platform in global defaults for image configs

[gustav-b]

Description

Thanks for adding #887 – it's a great feature! In addition to specifying watch and tag defaults, would it make sense to add platform too?

I'm currently using a YAML anchor/aliases to keep my file config DRY, but I was hoping to replace it with the new global defaults. What's holding me back is having to repeat the same platform-block on a twenty images (with just one exception). The host running diun has a different architecture than the images I want to monitor so I can't use the automatic option.

[crazy-max]

I was not sure about platform but I think that's fine to have it as well.