Docker instance's environment variables (GITEA__xxx) are not respected in 1.20-rc1

Regression of #25330 :

Run Gitea's docker image:

```
...s/setting/setting.go:180:loadRunModeFrom() [F] Gitea is not supposed to be run as root. Sorry. If you need to use privileged TCP ports please instead use setcap and the `cap_net_bind_service` permission
```



### Gitea Version

1.20

### How are you running Gitea?

Docker
