docs(installation): add HOST/port and Cloudflare Tunnel guidance

[mahdi13830510]

Summary

Addresses #2035 and #2195

Adds two new sections to the Docker Compose installation guide:

HOST / port configuration

• Explains why localhost must not be used as HOST inside Docker
• Shows how to change the listening port via ONEUPTIME_HTTP_PORT
• Reminds users to update probe URLs when changing the port

Cloudflare Tunnel setup

• Provides a working config.env snippet for Tunnel deployments
• Shows the two public hostnames to create in the Cloudflare Zero Trust dashboard
• Explains how to route custom-domain status pages via Cloudflare Custom Hostnames

Test plan

• Review documentation accuracy against docker-compose networking

[simlarsen]

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