Add containerized build/test recipe for contributors

[AnnabelJoe]

Category: DevOps

Description: Provide a reproducible containerized dev/test environment for contributors with mismatched local tooling.

Acceptance Criteria:

• A Dockerfile or script is available for builds/tests
• Contributors can run CI-like commands inside the container
• Instructions are documented

Priority: Low
Estimated Effort: Medium
Dependencies: container tooling
Suggested Labels: devops, developer-experience