Dockerfile上的错误指令COPY

我正在使用Dockerfile在Docker上构建MYSQL映像。我需要帮助，因为我有此问题：无法加载身份验证插件“ caching_sha2_password”

Aman Aggarwal的解决方案适合我，但我需要在自定义my.cnf上编写它，并尝试使用它重写my.cnf默认值，但是在执行docker build时出现错误：

COPY失败：stat /var/lib/docker/tmp/docker-builder204357196/my.cnf：没有这样的文件或目录

这是我的自定义my.cnf：

# Copyright (c) 2017, Oracle and/or its affiliates. All rights reserved.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; version 2 of the License.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA

#
# The MySQL  Server configuration file.
#
# For explanations see
# http://dev.mysql.com/doc/mysql/en/server-system-variables.html

[mysqld]
default_authentication_plugin = mysql_native_password
pid-file        = /var/run/mysqld/mysqld.pid
socket          = /var/run/mysqld/mysqld.sock
datadir         = /var/lib/mysql
secure-file-priv= NULL
# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0

# Custom config should go here
!includedir /etc/mysql/conf.d/

这是我的Dockerfile：

# Indicates that the mysql image will be used as the base image.
FROM mysql

#Try copy my.cnf on this path /etc/mysql/
COPY my.cnf /etc/mysql/my.cnf

#Try only to restart mysql (not container)
RUN service mysql restart

我的文件夹结构：

proyectos(folder)
  ->build_docker
      ->Dockerfile
      ->my.cnf