Fix#31361, and add tests
And this PR introduces an undocumented & debug-purpose-only config
option: `USE_SUB_URL_PATH`. It does nothing for end users, it only helps
the development of sub-path related problems.
And also fix#31366
Co-authored-by: @ExplodingDragon
Fix#31002
1. Mention Make sure `Host` and `X-Fowarded-Proto` headers are correctly passed to Gitea
2. Clarify the basic requirements and move the "general configuration" to the top
3. Add a comment for the "container registry"
4. Use 1.21 behavior if the reverse proxy is not correctly configured
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
