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>

.dockerignore

@@ -0,0 +1,26 @@
+# Build artifacts (cached in named volume instead)
+build-wasm/
+
+# Git internals (large, not needed for build)
+.git/modules/
+
+# Compiled files
+*.wasm
+*.o
+*.a
+*.o.tmp
+
+# Node modules (for tests)
+node_modules/
+
+# Output directory
+output/
+
+# IDE and editor files
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# macOS
+.DS_Store