Convert builder images to go

Builder images converted to go commands. Main builder code in pkg/build/builder
Mutauting objects like this is generally frowned on - why aren't these passing, mutating, or returning objects?  Seems like this would be better organized as

1. Create and return base obj
2. Extract Y from X and add it to the obj we just got
3. Etc

As is it's not clear what is doing what, and what the flow of info is.