Add docker-compose setup and configurable host domains

[schlivo]

Summary

Adds Docker Compose setup for development and makes host domain configuration flexible via environment variables, enabling remote containerized development and production deployments.

Changes

1. Docker Compose Setup

• Added docker-compose.yml with MailHog for email development
• Replaces console email output with MailHog web UI for better email testing
• Enables containerized development with hot-reload support

2. Configurable Host Domains

• Replaced hardcoded domain patterns with ALLOWED_HOST_DOMAINS environment variable
• Added support for custom domains in both development and production environments
• Enables remote development and production container deployments

3. Documentation

• Updated README.md with Docker Compose usage and remote development instructions
• Updated config/deploy.yml with ALLOWED_HOST_DOMAINS configuration example

Why

• Makes the codebase more flexible for different deployment scenarios
• Enables proper email testing during development via MailHog
• Allows remote containerized development without modifying code
• Follows the same configuration pattern as #1911 (environment variable-based configuration)

Testing

• Tested Docker Compose setup with MailHog email capture
• Verified remote hostname access via ALLOWED_HOST_DOMAINS environment variable
• Confirmed subdomain matching works correctly for configured domains

[flavorjones]

cc @kevinmcconnell, related to improving OSS deployment

[kevinmcconnell]

Replaces console email output with MailHog web UI for better email testing

Note that we do currently have letter_opener for testing emails. You can enable that with bin/rails dev:email, and then any emails sent in development will open in the browser automatically.