dockerfile的脚本内容输入后，docker build -t xbf/hello_nginx这条命令输入后报错，求解

Question

docker build -t xbf/hello_nginxinvalid argument "xbf/hello_nginx." for t: Error parsing reference: "xbf/hello_nginx." is not a valid repository/tag: invalid reference formatSee 'docker build --help'.我的脚本如下FROM ubuntuMAINTAINER xbfRUN sed -i 's/archive.ubuntu.com/mirrors.ustc.edu.cn/g' /etc/apt/sources.listRUN apt-get updateRUN apt-get install -y nginxCOPY index.html /var/www/htmlENTRYPOINT ["/usr/sbin/nginx","-g","daemon off:"]EXPOSE 80不知道到底哪有问题