added header for shell script

Signed-off-by: hillium <[email protected]>

images/ebs-warmup/warmup-steps.sh

@@ -1,4 +1,16 @@
 #! /usr/bin/env sh
+# Copyright 2023 PingCAP, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# See the License for the specific language governing permissions and
+# limitations under the License.
 
 set -eu