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>

tests/playwright.config.ts

@@ -1,4 +1,49 @@
 import { defineConfig, devices } from '@playwright/test';
+import { execSync } from 'child_process';
+import * as fs from 'fs';
+import * as path from 'path';
+
+const PORT_FILE = path.join(__dirname, '.test-port');
+
+// Get existing port from file or find a new one
+// This ensures all workers use the same port
+function getOrFindPort(): number {
+  // Check if port file exists and is recent (created in last 60 seconds)
+  try {
+    const stat = fs.statSync(PORT_FILE);
+    const age = Date.now() - stat.mtimeMs;
+    if (age < 60000) {
+      const port = parseInt(fs.readFileSync(PORT_FILE, 'utf-8').trim());
+      if (port > 0 && port < 65536) {
+        return port;
+      }
+    }
+  } catch {
+    // File doesn't exist or can't be read
+  }
+
+  // Find a new free port
+  const port = findFreePort();
+  fs.writeFileSync(PORT_FILE, port.toString());
+  return port;
+}
+
+// Find a free port dynamically using a shell command
+function findFreePort(): number {
+  // Use Python to find a free port (works on macOS and Linux)
+  try {
+    const result = execSync(
+      'python3 -c "import socket; s=socket.socket(); s.bind((\'\',0)); print(s.getsockname()[1]); s.close()"',
+      { encoding: 'utf-8' }
+    );
+    return parseInt(result.trim());
+  } catch {
+    // Fallback to default port range
+    return 9000 + Math.floor(Math.random() * 1000);
+  }
+}
+
+const port = getOrFindPort();
 
 export default defineConfig({
   testDir: './e2e',
@@ -13,7 +58,7 @@ export default defineConfig({
   testIgnore: ['**/button-finder.spec.ts'],
 
   use: {
-    baseURL: 'http://localhost:8080',
+    baseURL: `http://localhost:${port}`,
     trace: 'on-first-retry',
     // Grant clipboard and font permissions for tests
     permissions: ['clipboard-read', 'clipboard-write', 'local-fonts'],
@@ -27,8 +72,8 @@ export default defineConfig({
   ],
 
   webServer: {
-    command: 'npx serve wasm-app -p 8080',
-    port: 8080,
+    command: `npx serve wasm-app -p ${port}`,
+    port: port,
     reuseExistingServer: !process.env.CI,
   },
 });