[skip e2e] Add comment for set_docker_mirror.sh (#14401)

Signed-off-by: Jenny Li <jing.li@zilliz.com>

build/set_docker_mirror.sh

@@ -6,6 +6,8 @@ set -e
 
 # Use nexus as docker mirror registry
 MIRROR_URL="http://nexus-nexus-repository-manager-docker-5000.nexus:5000"
+
+# Add registry mirror config into docker daemon
 set_daemon_json_file(){
     DOCKER_DAEMON_JSON_FILE="/etc/docker/daemon.json"
     mkdir -p "/etc/docker"