Next uses invalid URL behind proxy

Link to the code that reproduces this issue

https://github.com/apfelbox/reproducer-invalid-next-host

To Reproduce

1. Run pnpm dev
2. Open http://localhost:3000
3. Open the same URL via a reverse proxy.

Current vs. Expected behavior

The internal URL (in request.url and request.nextUrl) should use the correct and proper URL, as otherwise we can't use it for anything output related. We can't just generate an URL, as it will be generated with localhost:3000.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.0.0: Fri Sep 15 14:41:43 PDT 2023; root:xnu-10002.1.13~1/RELEASE_ARM64_T6000
Binaries:
  Node: 18.19.0
  npm: 10.2.3
  Yarn: N/A
  pnpm: 8.15.5
Relevant Packages:
  next: 14.1.3
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.4.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

App Router

Which stage(s) are affected? (Select all that apply)

next dev (local), next start (local)

Additional context

No response

Start with the linked reproducer and run pnpm dev, checking request.url and request.nextUrl both directly and through a reverse proxy. Trace how the App Router constructs these URLs in next dev and next start. Done means both values use the proper proxied URL instead of localhost:3000.