toad serve and port remapping/reverse proxies

[rcarmo]

Toad serve doesn't work when served from inside Docker, maybe because of Textual URL checks:

• toad inside docker runs with --port 3000 --host 0.0.0.0
• docker remaps 3000 to 3004 on the host
• http://host:3004 only shows the initial loading screen, never gets past that

[rcarmo]

I think this is because public_url can't be set in toad serve, given what I'm seeing in the browser console.

[willmcgugan]

I'd accept a PR for that.