What package manager are you using?

yarn

What operating system are you using?

Mac

Describe the Bug

You can see in astro/docs that Astro.site resolves to https://docs.astro.build/ when running the dev server, rather than localhost:3000, which is what I would expect Astro.site to resolve to, similar to Hugo

Steps to Reproduce

1. clone astro project
2. yarn
3. yarn build
4. yarn workspace docs start
5. Print out console.debug(Astro.site):

URL {
  href: 'https://docs.astro.build/',
  origin: 'https://docs.astro.build',
  protocol: 'https:',
  username: '',
  password: '',
  host: 'docs.astro.build',
  hostname: 'docs.astro.build',
  port: '',
  pathname: '/',
  search: '',
  searchParams: URLSearchParams {},
  hash: ''
}

console.debug(Astro.site.toString()) => https://docs.astro.build/

@chenxsan sorted out that it's probably set in https://github.com/snowpackjs/astro/blob/main/packages/astro/src/runtime.ts#L93

Link to Minimal Reproducible Example (Optional)

No response