docs(server): clarify compose bridge networking and proxy usage

[liuxiaopai-ai]

Summary

• add a Docker Compose deployment note for opensandbox-server running inside containerized bridge networks
• document [docker].host_ip usage and why it is required for host-reachable endpoint resolution
• add use_server_proxy=true API example for clients that cannot directly access sandbox bridge addresses
• update both English and Chinese server README files

Why

Issue #252 reports unreachable endpoints in containerized bridge deployments. This PR improves deployment guidance so operators can configure reachable endpoints and proxy usage correctly.

Closes #252

Validation

• documentation-only change
• examples align with server/docker-compose.example.yaml

[CLAassistant]

All committers have signed the CLA.