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/Dockerfile

@@ -0,0 +1,25 @@
+# Use ARM64-native image for Apple Silicon (M1/M2/M3/M4)
+FROM emscripten/emsdk:4.0.2-arm64
+
+# Install build tools required for KiCad WASM build
+RUN apt-get update && apt-get install -y \
+    cmake \
+    autoconf \
+    automake \
+    libtool \
+    meson \
+    ninja-build \
+    pkg-config \
+    curl \
+    git \
+    python3 \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /workspace
+
+# Entry point that sources Emscripten environment
+COPY docker/entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+ENTRYPOINT ["/entrypoint.sh"]
+CMD ["bash"]