✨ Add public-url option and env var

[YuriiMotov]

fastapi run command output includes URLs of server and docs:

    server   Server started at http://0.0.0.0:8000
    server   Documentation at http://0.0.0.0:8000/docs

The 0.0.0.0 in URL here doesn't make sense - links don't work..

This PR adds --public-url option and FASTAPI_PUBLIC_URL env variable to configure the base of these URLs:

$ fastapi run --public-url https://myapp.example.com/api

    server   Server started at https://myapp.example.com/api
    server   Documentation at https://myapp.example.com/api/docs

---

We can also describe this config option in the readme:

Details

## Public URL

When you run behind a reverse proxy or in a container - typically when binding to `0.0.0.0` - the URLs printed at startup (`http://0.0.0.0:8000/docs`) aren't the URLs that users actually use to reach the server.

You can use `--public-url` (or the `FASTAPI_PUBLIC_URL` environment variable) to tell the CLI which URL to display:

<div class="termy">

```console
$ fastapi run main.py --public-url https://api.example.com

   FastAPI   Starting production server 🚀

             ...

    server   Server started at https://api.example.com
    server   Documentation at https://api.example.com/docs
```

</div>

This flag is display-only - it doesn't change the host, port, or routing. Set it to the address your users (or your proxy's public frontend) actually reach.

[Copilot]

Pull request overview

Adds a CLI-level way to override the “public” base URL shown in fastapi dev / fastapi run startup output, so the printed server/docs links are usable when binding to 0.0.0.0 or running behind a proxy.

Changes:

• Add --public-url option (and FASTAPI_PUBLIC_URL env var) to fastapi dev and fastapi run.
• Use public_url (normalized for trailing /) to build the displayed server/docs URLs.
• Add CLI tests and help-output assertions covering the new option/env var.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
src/fastapi_cli/cli.py	Adds public_url plumbing and uses it to form the displayed startup URLs; exposes the option/env var on dev and run.
tests/test_cli.py	Adds tests for --public-url and FASTAPI_PUBLIC_URL, plus help text assertions.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[YuriiMotov]

I think we can ignore this for now and add such guard if user report they need this (I think they will not)

[YuriiMotov]

Not sure we need this