### Description
This PR fixes a bug where Nginx fails to start when using the
`ragflow.https.conf` configuration. The upstream host `ragflow` was not
resolving correctly inside the container context, causing an `[emerg]
host not found` error.
### Changes
- Updated `docker/nginx/ragflow.https.conf`: Changed upstream host from
`ragflow` to `localhost` for both the admin API and the main API.
### Related Issue
Fixes#11453
### Testing
- [x] Enabled HTTPS config in Docker.
- [x] Verified Nginx starts successfully without "host not found"
errors.
- [x] Verified API accessibility.
### What problem does this PR solve?
_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._
### Type of change: Documentation Update/Refactoring
#### Summary
Adds HTTPS/SSL configuration guide/example to enable secure RAGFlow
deployments with proper certificate management.
#### Changes
- New HTTPS Setup Section: Step-by-step guide for SSL certificate
configuration
- Let's Encrypt Integration: Complete Certbot setup instructions
- Docker Configuration: Volume mapping examples for certificates
#### Key Features
- Prerequisites checklist
- Docker Compose configuration examples
- Support for both Let's Encrypt and existing certificates
#### Files Modified
- `README.md`
- `ragflow.https.conf` (new file)