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/README.md

@@ -0,0 +1,96 @@
+# Docker Build Environment for KiCad WASM
+
+This directory contains Docker configuration for building KiCad for WebAssembly in a reproducible, isolated environment.
+
+## Prerequisites
+
+- Docker Desktop (with Docker Compose v2)
+- Git submodules initialized: `git submodule update --init --recursive`
+
+## Quick Start
+
+```bash
+# Build KiCad WASM (full build)
+./docker/build.sh
+
+# Build with options
+./docker/build.sh --no-clean    # Skip cleaning build directory
+./docker/build.sh --debug       # Build with debug symbols
+
+# Interactive shell for debugging
+./docker/shell.sh
+```
+
+## What Gets Built
+
+The build process includes:
+
+1. **Dependencies** (cached in Docker volume):
+   - GLM (header-only)
+   - Zstd, Protobuf
+   - FreeType, HarfBuzz
+   - Pixman, Cairo
+   - Boost (Locale)
+   - OpenCASCADE (optional, for 3D/STEP)
+   - CURL headers, libgit2 headers (stubs)
+
+2. **wxWidgets** (built from submodule)
+
+3. **KiCad PCBnew** (main application)
+
+## Container Resources
+
+Configured for M4 Max (adjust in docker-compose.yml):
+- CPUs: 10 cores
+- Memory: 16GB
+
+## Volume Strategy
+
+| Path | Type | Purpose |
+|------|------|---------|
+| `/workspace` | Bind mount | Source code |
+| `/workspace/build-wasm` | Named volume | Build cache (deps, sysroot) |
+| `/workspace/output` | Bind mount | Final WASM output |
+
+## Common Commands
+
+```bash
+# Start container
+docker compose -f docker/docker-compose.yml up -d
+
+# Run a command inside
+docker compose -f docker/docker-compose.yml exec kicad-wasm-builder <command>
+
+# View logs
+docker compose -f docker/docker-compose.yml logs -f
+
+# Stop container
+docker compose -f docker/docker-compose.yml down
+
+# Clear build cache (full rebuild)
+docker volume rm docker_kicad-build-cache
+```
+
+## Troubleshooting
+
+### Build freezes
+The OpenCASCADE build is very resource-intensive. If it freezes:
+1. Reduce parallel jobs: Edit script to use `-j4` instead of `-j$(nproc)`
+2. Monitor with `docker stats`
+3. Consider building OpenCASCADE separately with `./scripts/deps/build-opencascade.sh`
+
+### Permission issues
+Files created in container are owned by root. To fix:
+```bash
+sudo chown -R $(whoami) output/
+```
+
+### Cache issues
+```bash
+# Clear all cached builds
+docker volume rm docker_kicad-build-cache
+
+# Or clear specific stamps inside container
+./docker/shell.sh
+rm /workspace/build-wasm/stamps/*.stamp
+```