Add Docker/local demo environment for quick evaluation

[httpdss]

Summary

Add a Docker-based local demo or test environment so contributors and new users can see collectd-web working without configuring a full system first.

Why this matters

A low-friction demo environment makes screenshots, Shorts, docs verification, and contributor onboarding much easier. It also gives curious users a quick way to evaluate the project.

Scope

• Provide a Dockerfile/docker-compose setup or documented equivalent.
• Include sample collectd/RRD data or a script to generate demo data if licensing/size permits.
• Document how to start the demo and what URL to open.
• Clarify whether the demo is for evaluation only or supported deployment.

Acceptance criteria

• A contributor can run one command to start a local demo or test server.
• Demo renders at least one graph/dashboard view.
• README or docs link to the demo instructions.
• Any sample data is small and appropriate for the repo.