我的是WSL2子系统，当前网络的DNS服务器发生变化了
执行修改了 /etc/host 文件，wq！以后还是会被还原。提示为：

# This file was automatically generated by WSL. To stop automatic generation of this file, add the following entry to /etc/wsl.conf:
# [network]
# generateHosts = false

遂执行

sudo vi /etc/wsl.conf

添加如下内容：

[network]
generateHosts = false

vi /etc/resolv.conf

添加红色框里边的三条DNS服务器解析地址，复制放到/etc/resolv.conf添加覆盖之前的就行。
 

然后执行

source ~/.bashrc

最后测试

docker search mysql

此时若不打开科学上网络，返回

root@xxxxx:~# docker search mysql
Error response from daemon: Get "https://index.docker.io/v1/search?q=mysql&n=25": dialing index.docker.io:443 container via direct connection because  has no HTTPS proxy: connecting to index.docker.io:443: dial tcp 31.13.87.9:443: connectex: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

此时若打开科学上网络，返回

root@xxxx:~# docker search mysql
NAME                   DESCRIPTION                                     STARS     OFFICIAL
mysql                  MySQL is a widely used, open-source relation…   15636     [OK]
bitnami/mysql          Bitnami container image for MySQL               130
circleci/mysql         MySQL is a widely used, open-source relation…   31
bitnamicharts/mysql    Bitnami Helm chart for MySQL                    0
cimg/mysql                                                             3
ubuntu/mysql           MySQL open source fast, stable, multi-thread…   67
linuxserver/mysql      A Mysql container, brought to you by LinuxSe…   41
google/mysql           MySQL server for Google Compute Engine          25
elestio/mysql          Mysql, verified and packaged by Elestio         1
docksal/mysql          MySQL service images for Docksal - https://d…   0
alpine/mysql           mysql client                                    3
mysql/mysql-server     Optimized MySQL Server Docker images. Create…   1026
jumpserver/mysql                                                       1
datajoint/mysql        MySQL image pre-configured to work smoothly …   2
ddev/mysql             ARM64 base images for ddev-dbserver-mysql-8.…   1
mysql/mysql-router     MySQL Router provides transparent routing be…   28
mirantis/mysql                                                         0
ilios/mysql            Mysql configured for running Ilios              1
corpusops/mysql        https://github.com/corpusops/docker-images/     0
mysql/mysql-cluster    Experimental MySQL Cluster Docker images. Cr…   100
vulhub/mysql                                                           1
javanile/mysql         MySQL for development                           0
mysql/mysql-operator   MySQL Operator for Kubernetes                   1
vitess/mysql           Lightweight image to run MySQL with Vitess      1
nasqueron/mysql                                                        1

至此，问题已得到解决！