markdown: wrong hostname in commit url with empty ROOT_URL

[milahu]

Description

when i leave ROOT_URL empty
then gitea uses http://localhost as hostname in commit urls
but gitea should produce urls without hostname

ping @KN4CK3R via #28909

repro https://try.gitea.io/milahu/test-gitea-markdown-wrong-commit-url

part of #19345

Gitea Version

1.21.6

Can you reproduce the bug on the Gitea demo site?

Yes

Log Gist

No response

Screenshots

No response

Git Version

No response

Operating System

No response

How are you running Gitea?

gitea runs in a container on localhost, accessed from localhost
so gitea thinks that its hostname is "localhost", but that is wrong

Database

None

[lunny]

ROOT_URL is almost a MUST set option especially if you run gitea on the container.

[milahu]

ROOT_URL is almost a MUST set option

that is the problem...
the same gitea instance should work on localhost and somewhere.com
and the localhost version should have no links to somewhere.com

this already works with file links, only commit links are broken

[milahu]

in my gitea config, both DOMAIN and ROOT_URL are empty
so gitea should use no hostname in urls, just absolute paths like /explore/repos

[server]
DOMAIN =
ROOT_URL =

when urls with hostname are required
then gitea should use the actual hostname and port from the http request
but usually, browsers are smart enough to infer the full url from absolute paths

only commit links are broken

there are some other places where gitea uses localhost:12345

<link rel="manifest" href="data:application/json;base64,...">

decoded manifest json

{
  "name": "test",
  "short_name": "test",
  "start_url": "http://localhost:12345/",
  "icons": [
    {
      "src": "http://localhost:12345/assets/img/logo.png",
      "type": "image/png",
      "sizes": "512x512"
    },
    {
      "src": "http://localhost:12345/assets/img/logo.svg",
      "type": "image/svg+xml",
      "sizes": "512x512"
    }
  ]
}

<script>
	// ...
	// the escaped slashes look wrong
	window.config = {
		appUrl: 'http:\/\/localhost:12345\/',

<meta property="og:url" content="http://localhost:12345/someone/something">

<button class="ui small compact clone button" id="repo-clone-https" data-link="http://localhost:12345/someone/something.git">
	HTTPS
</button>

<input id="repo-clone-url" size="20" class="js-clone-url" value="http://localhost:12345/someone/something.git" readonly>

[KN4CK3R]

Gitea is currently not build to be fully domain independend and may never be. Some parts (like the commit url) can be transformed but that's not possible everywhere. Here is an issue with more information: #19345