feat: improve install script and test containers handling
- Extract install script logic into testable functions and add unit tests for each
- Add case-insensitive target selection to select_targets
- Add docker/kill-if-running to detect and kill previous test containers with warning
- Include docker.mk in core targets so docker utilities are always available
- Remove docker from available install targets (now always included)
Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com