contrib/sni-router: switch env alias to merge key

Lets downstreams add per-service env vars (e.g. HOST for Caddy's
Dynu DNS-01 challenge) without breaking the shared DOMAIN alias.

contrib/sni-router/docker-compose.yml

       - "80:80"
     volumes:
       - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg:ro,Z
-    environment: *domain-env
+    environment:
+      <<: *domain-env
     depends_on:
       - mtg
       - web
     expose:
       - "80"
       - "8443"
-    environment: *domain-env
+    environment:
+      <<: *domain-env
     restart: unless-stopped
 
 volumes: