Whether you'd call "composition-first" is probably asking for a big argument about what "composition first" really means, but Go is certainly a language that syntactically privileges a particular type of composition over inheritance. It doesn't even have syntax for inheritance, and frankly even manually implementing it is rather a pain (best I've ever done requires you to pass the "object" as a separate parameter to every method call... and, yes, I said that correctly, to every method call).
I'm not ready to try to stake a position on the top of some "composition first" hill because the syntactic composition it supports is not something I use all the time. It's an occasional convenience more than a fundamental primitive in the language, the way inheritance is in inheritance-based languages. Most of the composition is just done through methods that happen to use in composed-in values, but it is generally not particularly supported by syntax.
I'm not ready to try to stake a position on the top of some "composition first" hill because the syntactic composition it supports is not something I use all the time. It's an occasional convenience more than a fundamental primitive in the language, the way inheritance is in inheritance-based languages. Most of the composition is just done through methods that happen to use in composed-in values, but it is generally not particularly supported by syntax.