feat: multi-printer Docker Compose stack

[reneleban]

Problem

SentryBridge currently supports exactly one printer per container instance. Users with multiple printers need to run separate stacks.

Scope

• Documented Docker Compose template for multiple SentryBridge containers
• Different ports per instance via PORT environment variable
• No shared state between instances (architecture decision remains: one container = one printer)

Out of Scope

• Multiple printers in a single container
• Central dashboard across multiple instances