contrib/sni-router: deduplicate DOMAIN env via YAML anchor

contrib/sni-router/docker-compose.yml

 #
 # See BEST_PRACTICES.md and the project wiki for background.
 
+x-domain-env: &domain-env
+  DOMAIN: ${DOMAIN:-example.com}
+
 services:
   haproxy:
     image: haproxy:lts-alpine
       - "80:80"
     volumes:
       - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg:ro,Z
-    environment:
-      DOMAIN: ${DOMAIN:-example.com}
+    environment: *domain-env
     depends_on:
       - mtg
       - web
     expose:
       - "80"
       - "8443"
-    environment:
-      DOMAIN: ${DOMAIN:-example.com}
+    environment: *domain-env
     restart: unless-stopped
 
 volumes: