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>

docker/shell.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+# Open interactive shell in build container for debugging
+set -e
+
+cd "$(dirname "$0")/.."
+
+# Start container if not running
+docker compose -f docker/docker-compose.yml up -d
+
+# Open interactive shell
+docker compose -f docker/docker-compose.yml exec kicad-wasm-builder bash