Docs: narrow the README dependency claim

[SandObserver]

Summary

• Reword the README dependency line: no runtime npm dependencies, and the image adds only nginx and supervisor from Alpine to the Node base.
• Add repository tests that pin the line, the empty API dependency list and the packages the Dockerfile installs.

Testing
All green. Added 3 repository tests.

Changelog

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.