Add healthcheck and start-first update for l1-server

Zero-downtime deploys: new container starts and passes health
check before the old one is stopped. Caddy always has a healthy
backend to proxy to.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -75,6 +75,12 @@ services:
       - CACHE_DIR=/data/cache
       # DATABASE_URL, ADMIN_TOKEN, ARTDAG_CLUSTER_KEY,
       # L2_SERVER, L2_DOMAIN, IPFS_GATEWAY_URL from .env file
+    healthcheck:
+      test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8100/')"]
+      interval: 10s
+      timeout: 5s
+      retries: 3
+      start_period: 15s
     volumes:
       - l1_cache:/data/cache
     depends_on:
@@ -86,6 +92,8 @@ services:
       - externalnet
     deploy:
       replicas: 1
+      update_config:
+        order: start-first
       restart_policy:
         condition: on-failure
       placement: