fix: host-based mapping with local backend

[HolgerHuo]

fix host-based mapping to local backend not working

[HolgerHuo]

This pr fixes local backend cannot be used in img_map. However, there's still room for further improvements:

1. Maybe we should handle url parsing of remote backends in config.go so that they don't have to be parsed on every request
2. The ProxyMode switch should be fully deprecated, and this may require some in-depth updates to the codebase.

This PR should be ready for merge for now. Should I have time to implement the above mentioned improvements, I'll put them in another PR.

P.S.: The failed workflow should be an apt issue. A restart of the workflow run should fix it.

[HolgerHuo]

Seems still buggy in getId() 😂 I'll try fix it later