Extract Docker helpers and host/Docker branching to mk/core/docker.mk

[pbalduino]

Parent Issue

Part of #352 - Makefile refactoring

Objective

Centralize Docker-only helpers and host/Docker branching logic repeated across pattern rules.

Scope

Extract from root Makefile:

• Lines ~307: Docker helper functions
• Lines ~328: Host/Docker conditional logic
• Lines ~453: Pattern rule Docker branching
• Wrapper macros/functions for Docker execution

Target Location

mk/core/docker.mk

Implementation Steps

1. Extract Docker detection and helper functions
2. Create wrapper macros for Docker-vs-host execution
3. Update root Makefile to include mk/core/docker.mk
4. Refactor pattern rules to use centralized helpers
5. Test both host and Docker builds

Dependencies

None - this is a foundational module

Acceptance Criteria

• [ ] All Docker helpers centralized in mk/core/docker.mk
• [ ] No duplicated Docker branching logic
• [ ] Both host and Docker builds work correctly
• [ ] Pattern rules simplified using new helpers