I would say that the adjective "готово" was created from the full form of the adjective "готовый", but in the sentence "молоко готово" it plays the role of a predicate (usually performed by a verb). The predicate expressed by a short form of an adjective characterizes the subject. For example, "Our puppy is playful" will be translated as "Наш щенок игрив" (the full form is "игривый"). The difference between a short and a full form of an adjective is that the short form is used only in the form of a predicate, never as an attribute, except some folk-style texts. Full forms can be used as both an attribute and a predicate: "Игривый щенок сейчас на улице" - "Наш щенок игривый".
There are also some shades of meanings when one selects which form to use: the short form of an adjective as a rule means something temporary, that exact moment condition or feeling regarding some event: "Рыбалка была хороша!" (the fishing that day was good) vs. "Рыбалка тут хорошая" (it ism in general, good place for fishing). So, the characteristic of something/somebody one expresses using a short form of an adjective, is restricted by some circumstances.
