Add Tailnet search domain to container DNS config

The custom dns servers added in 2d49dab caused Docker to drop the
Tailscale search domain from the container's resolv.conf, breaking
short hostname resolution (e.g. pb-dev). Adding dns_search with
TAILNET_DOMAIN restores it.

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

infmap/docker-compose.yml

@@ -4,6 +4,8 @@ services:
     dns:
       - 100.100.100.100
       - 8.8.8.8
+    dns_search:
+      - ${TAILNET_DOMAIN}
     ports:
       - "${WEB_PORT}:5000"
     environment: