Add DNS resolver for automatic container IP re-resolution

When Docker containers restart, they can get new IPs on the bridge
network. HAProxy caches DNS at config load time, so stale IPs cause
503s until config is regenerated.

Added a 'docker_dns' resolvers section pointing to Docker's embedded
DNS (127.0.0.11) with 10s hold time. Backend servers now use
'resolvers docker_dns init-addr last,libc,none' so HAProxy:
- Re-resolves container names every 10 seconds
- Falls back to last known IP if DNS is temporarily unavailable
- Starts even if a backend can't be resolved yet (init-addr none)

This eliminates 503s from container restarts, scaling, and recreation
without requiring a HAProxy config regeneration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/hap_backend.tpl

@@ -11,7 +11,7 @@ backend {{ name }}-backend
     http-request set-header X-Forwarded-Proto http if !{ ssl_fc }
 
     {% for server in servers %}
-    server {{ server.server_name }} {{ server.server_address }}:{{ server.server_port }} {{ server.server_options }}
+    server {{ server.server_name }} {{ server.server_address }}:{{ server.server_port }} {{ server.server_options }} resolvers docker_dns init-addr last,libc,none
     {% endfor %}
 
 # SSE-specific backend - optimized for Server-Sent Events long-lived connections
@@ -36,5 +36,5 @@ backend {{ name }}-sse-backend
     http-request set-header X-Forwarded-Proto http if !{ ssl_fc }
 
     {% for server in servers %}
-    server {{ server.server_name }} {{ server.server_address }}:{{ server.server_port }} {{ server.server_options }}
+    server {{ server.server_name }} {{ server.server_address }}:{{ server.server_port }} {{ server.server_options }} resolvers docker_dns init-addr last,libc,none
     {% endfor %}