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