docs: add Docker Apple Silicon platform note

[sionsmith]

Task

Document that the published launch Docker image is Linux amd64, and show Apple Silicon users how to run it with --platform linux/amd64 until a multi-arch image is published.

Likely files

• README.md
• deploy/docker-compose.yaml
• chukei-docs Docker deployment page

Acceptance criteria

• Docker install docs mention the platform constraint.
• Example command is copy-pastable.
• No private account locators or credentials are added.