How to properly set site canonical URL?

[gcb]

When my app router application's route.ts file issue a return NextResponse.redirect(new URL('/', req.url))

it generates a redirect header to "http://localhost:3000/"... because that's where node is serving the side.

But in any sensible deployment, you are serving from https://example.com and nginx, or caddy, or some other proxy is terminating the https request and proxying to localhost.

What is the correct way to tell nextjs about the canonical url it must use for those headers that require full urls instead of relative?

The internet is litered with bad advice on this.