Add Docker deployment with automatic HTTPS

A two-stage image runs the tests and builds the game with Node, then
serves dist/ with Caddy, which obtains and renews Let's Encrypt
certificates for robotmarkets.ir (configurable via SITE_ADDRESS).
HTTPS is required for installing the PWA and for GPS mode.

- Caddyfile: gzip/zstd, long-lived caching for hashed assets, no-cache
  for the page, service worker and manifest, SPA fallback, permissions
  policy for location and motion sensors
- docker-compose.yml: ports 80/443, persistent certificate volumes
- README: server setup, updates, running behind an existing proxy, and
  workarounds when Docker Hub is unreachable from servers in Iran

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,20 @@
+services:
+  game:
+    build: .
+    image: robot-market-game:latest
+    restart: unless-stopped
+    ports:
+      - "80:80"
+      - "443:443"
+      - "443:443/udp"
+    environment:
+      # Point these domains' DNS A records at the server before the first start.
+      SITE_ADDRESS: ${SITE_ADDRESS:-robotmarkets.ir, www.robotmarkets.ir}
+    volumes:
+      # Keeps the HTTPS certificates across restarts and image updates.
+      - caddy_data:/data
+      - caddy_config:/config
+
+volumes:
+  caddy_data:
+  caddy_config: