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/docker-compose.yml

@@ -0,0 +1,28 @@
+services:
+  kicad-wasm-builder:
+    build:
+      context: ..
+      dockerfile: docker/Dockerfile
+    container_name: kicad-wasm-builder
+
+    # Resource limits (M4 Max: 10 cores, 16GB)
+    deploy:
+      resources:
+        limits:
+          cpus: '10'
+          memory: 16G
+
+    volumes:
+      # Source code (read-write for git operations)
+      - ..:/workspace:cached
+      # Named volume for build cache (faster than bind mount on macOS)
+      - kicad-build-cache:/workspace/build-wasm
+      # Output directory for easy access to final WASM files
+      - ../output:/workspace/output
+
+    # Keep container running for interactive use
+    stdin_open: true
+    tty: true
+
+volumes:
+  kicad-build-cache: