Bug#79374 user mysql.sys is removed from docker deployment

The user is needed for the sys schema. Changed script to preserve it.

Author: ltangvald

5.7/docker-entrypoint.sh

@@ -50,7 +50,7 @@ if [ "$1" = 'mysqld' ]; then
 			-- What's done in this file shouldn't be replicated
 			--  or products like mysql-fabric won't work
 			SET @@SESSION.SQL_LOG_BIN=0;
-			DELETE FROM mysql.user ;
+			DELETE FROM mysql.user where user != 'mysql.sys';
 			CREATE USER 'root'@'%' IDENTIFIED BY '${MYSQL_ROOT_PASSWORD}' ;
 			GRANT ALL ON *.* TO 'root'@'%' WITH GRANT OPTION ;
 			DROP DATABASE IF EXISTS test ;