I'm not sure if we should be really be having a go at them for whats on their git discussions verses whats in their documentation. I'd presume the documentation is canonical, I'd rather they weren't muting their discussions to remain consistent.

That said, as I said previously --build-args are dangerous, it's trivially easy to store then publish a secret, so it makes sense they weren't jumping for joy about implementing it. I'd say it is needed though, thus its now a thing.