It would be useful to be able to accept user input strings/fields or choose a related git reference when kicking off a custom build.
The user input could be made available as environmental variables. The names of the fields could be defined in the yml file, eg.
So far my use cases have all been doing things with two repos; so I could also use env vars that identify a repo, branch or tag based on user input (eg. dropdowns). That would have the nice side effect of checking the target exists.
I can already implicitly identify one side of that process by choosing where to trigger the build; but obviously not both. There are some ways to do build gymnastics and get around it, but it would be a heck of a lot easier to just be able to spec "this build needs that tag from that repo" when you run the custom build.