配置http代理拉取官方仓库镜像
参考文档：https://docs.docker.com/engine/daemon/proxy/

# 将http代理配置到shell环境变量
export http_proxy=http://192.168.6.88:10810
export HTTP_PROXY=http://192.168.6.88:10810
export HTTPS_PROXY=http://192.168.6.88:10810
export https_proxy=http://192.168.6.88:10810

# 通过拉取镜像发现docker并不会使用这个代理，重启docker服务也并不会加载代理的环境变量
[root@localhost ~]# docker pull nginx
Using default tag: latest
Error response from daemon: Get "https://registry-1.docker.io/v2/": net/http: request canceled while waiting for connection (Client.Timeout exceeded while awaiting headers)

# docker官方是3种配置方法
1、配置systemd插件（drop-in）
[root@localhost ~]# mkdir -p /etc/systemd/system/docker.service.d
[root@localhost ~]# vi /etc/systemd/system/docker.service.d/http-proxy.conf
[Service]
Environment="HTTP_PROXY=http://192.168.6.88:10810"
Environment="HTTPS_PROXY=http://192.168.6.88:10810"
Environment="NO_PROXY=localhost,127.0.0.1,.example.com"

[root@localhost ~]# systemctl daemon-reload
[root@localhost ~]# systemctl restart docker

# 查看是否加载
[root@localhost ~]# systemctl show docker --property=Environment
Environment=HTTP_PROXY=http://192.168.6.88:10810 HTTPS_PROXY=http://192.168.6.88:10810 NO_PROXY=localhost,127.0.0.1,.example.com

2、配置 daemon.json
[root@localhost ~]# vi /etc/docker/daemon.json

{
  "proxies": {
    "default": {
      "httpProxy": "http://192.168.6.88:10810",
      "httpsProxy": "http://192.168.6.88:10810",
      "noProxy": "localhost,127.0.0.1,.example.com"
    }
  }
}

[root@localhost ~]# systemctl restart docker

3、配置环境变量
Docker 守护进程在启动时，会检查以下环境变量实现代理
HTTP_PROXY
http_proxy
HTTPS_PROXY
https_proxy
NO_PROXY
no_proxy
在上面的实际测试时并没有效果，推测是因为systemd管理的docker服务因为不是在同一个shell环境启动，不会继承这些变量。
[root@localhost ~]# /usr/bin/dockerd -h | grep proxy
      --http-proxy string                     HTTP proxy URL to use for outgoing traffic
      --https-proxy string                    HTTPS proxy URL to use for outgoing traffic
      --no-proxy string                       Comma-separated list of hosts or IP addresses for which the proxy is skipped
通过帮助文档来看是可以通过传递参数实现。但需要手动启动服务，就不能用systemd管理了。

export http_proxy=http://192.168.6.88:10810
export HTTP_PROXY=http://192.168.6.88:10810
export HTTPS_PROXY=http://192.168.6.88:10810
export https_proxy=http://192.168.6.88:10810

/usr/bin/dockerd

这样手动启动的docker服务才会继承代理变量并生效。

# 带账号密码的代理写法
http://user:pass@192.168.6.88:10810

至此，可以通过镜像站、自建registry加速、配置代理 3种方式实现拉取官方镜像