Pass link prefixes to external markup parsers

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -359,6 +359,10 @@ IS_INPUT_FILE = false
 - RENDER\_COMMAND: External command to render all matching extensions.
 - IS\_INPUT\_FILE: **false** Input is not a standard input but a file param followed `RENDER_COMMAND`.
 
+Two special environment variables are passed to the render command:
+- `GITEA_PREFIX_SRC`, which contains the current URL prefix in the `src` path tree. To be used as prefix for links.
+- `GITEA_PREFIX_RAW`, which contains the current URL prefix in the `raw` path tree. To be used as prefix for image paths.
+
 ## Other (`other`)
 
 - `SHOW_FOOTER_BRANDING`: **false**: Show Gitea branding in the footer.

modules/markup/external/external.go

@@ -76,6 +76,11 @@ func (p *Parser) Render(rawBytes []byte, urlPrefix string, metas map[string]stri
 	}
 
 	cmd := exec.Command(commands[0], args...)
+	cmd.Env = append(
+		os.Environ(),
+		"GITEA_PREFIX_SRC="+urlPrefix,
+		"GITEA_PREFIX_RAW="+strings.Replace(urlPrefix, "/src/", "/raw/", 1),
+	)
 	if !p.IsInputFile {
 		cmd.Stdin = rd
 	}