docker 镜像启动时 会执行 entrypoint.sh 脚本；其中关于是否执行数据库初始化的核心代码如下：

   由脚本可以看出，对于docker-entrypoint-initdb.d 目录下的sql 文件，在执行前会判断  DATABASE_ALREADY_EXISTS 是否为false
即 $DATADIR/mysql 目录并不存在；但凡该服务器已经存在任何数据库，那么这个文件肯定不为空，故而不会执行sql脚本；

docker_setup_env() {
	# Get config
	declare -g DATADIR SOCKET
	DATADIR="$(mysql_get_config 'datadir' "$@")"
	SOCKET="$(mysql_get_config 'socket' "$@")"

	# Initialize values that might be stored in a file
	file_env 'MYSQL_ROOT_HOST' '%'
	file_env 'MYSQL_DATABASE'
	file_env 'MYSQL_USER'
	file_env 'MYSQL_PASSWORD'
	file_env 'MYSQL_ROOT_PASSWORD'

	declare -g DATABASE_ALREADY_EXISTS
	if [ -d "$DATADIR/mysql" ]; then
		DATABASE_ALREADY_EXISTS='true'
	fi
}


_main() {
        # if command starts with an option, prepend mysqld
        if [ "${1:0:1}" = '-' ]; then
                set -- mysqld "$@"
        fi

        # skip setup if they aren't running mysqld or want an option that stops mysqld
        if [ "$1" = 'mysqld' ] && ! _mysql_want_help "$@"; then
                mysql_note "Entrypoint script for MySQL Server ${MYSQL_VERSION} started."

                mysql_check_config "$@"
                # Load various environment variables
                docker_setup_env "$@"
                docker_create_db_directories

                # If container is started as root user, restart as dedicated mysql user
                if [ "$(id -u)" = "0" ]; then
                        mysql_note "Switching to dedicated user 'mysql'"
                        exec gosu mysql "$BASH_SOURCE" "$@"
                fi

                # there's no database, so it needs to be initialized
                if [ -z "$DATABASE_ALREADY_EXISTS" ]; then
                        docker_verify_minimum_env

                        # check dir permissions to reduce likelihood of half-initialized database
                        ls /docker-entrypoint-initdb.d/ > /dev/null

                        docker_init_database_dir "$@"
                        ......