Fix compose fallback variable

config/piler-ssl.yml

@@ -26,7 +26,7 @@ services:
       - /var/run/docker.sock:/var/run/docker.sock:ro
     environment:
       - LETSENCRYPT_EMAIL=${LETSENCRYPT_EMAIL}
-      - ENABLE_IPV6=${NGINX_ENABLE_IPV6:false}
+      - ENABLE_IPV6=${NGINX_ENABLE_IPV6:-false}
 
   mysql:
     image: mariadb:${MARIA_DB_VERSION}