fix: add curl and pass environment variables to Docker build

- Install curl in Alpine image for health checks
- Add ARG and ENV declarations for NEXT_PUBLIC_* variables in builder stage
- Pass build args from docker-compose.yml to Dockerfile
- Update health checks to use curl instead of node -e
- Update .env.example with helpful comments for Docker vs local dev

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker-compose.yml

@@ -5,6 +5,9 @@ services:
     build:
       context: .
       dockerfile: Dockerfile
+      args:
+        NEXT_PUBLIC_API_URL: ${NEXT_PUBLIC_API_URL:-http://localhost:3001}
+        NEXT_PUBLIC_APP_URL: ${NEXT_PUBLIC_APP_URL:-http://localhost:3000}
     image: ghcr.io/valknarness/pastel-ui:latest
     container_name: pastel-ui
     ports:
@@ -15,7 +18,7 @@ services:
       - NEXT_PUBLIC_APP_URL=${NEXT_PUBLIC_APP_URL:-http://localhost:3000}
     restart: unless-stopped
     healthcheck:
-      test: ["CMD", "node", "-e", "require('http').get('http://localhost:3000/', (r) => {process.exit(r.statusCode === 200 ? 0 : 1)})"]
+      test: ["CMD", "curl", "-f", "http://localhost:3000/"]
       interval: 30s
       timeout: 10s
       retries: 3