I had this problem too in earlier iterations of apps/frameworks which attempted to rewrite the signature of handlers; I've come to believe that replacing the current signature is a mistake, because it is too tempting to load your context with everything you might possibly need (this one has around 20 methods). As soon as an interface hits > 4 methods or so you may as well specify a concrete type, because you're specifying so much behaviour that it's really tied to one particular implementation. FWIW Buffalo has the same problem.
net/http doesn't actually supply everything you need to build a full-stack website though - you need authn, authz, csrf, assets, view rendering and a few other things, you can certainly build a minimal backend with it, but you need more in order to build a website.