Add Docker build environment for KiCad WASM

- Add Dockerfile with ARM64-native emscripten/emsdk:4.0.2-arm64 image
- Add docker-compose.yml with resource limits (10 CPUs, 16GB RAM)
- Add helper scripts (build.sh, shell.sh, entrypoint.sh)
- Add Docker README with usage instructions
- Update playwright.config.ts to find free port dynamically
- Add .dockerignore and .gitignore entries for build artifacts

The Docker environment provides reproducible builds with:
- Named volume for build cache (faster I/O on macOS)
- Resource limits to prevent system lockups
- Interactive shell access for debugging

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -37,6 +37,7 @@ wxwidgets-clean/
 /tests/node_modules/
 /tests/playwright-report/
 /tests/test-results/
+/tests/.test-port
 /test-results/
 /tests/wasm-app/*.js
 /tests/wasm-app/*.html