ubuntu更新源为阿里云

1. /etc/apt/sources.list
   官方网站对每个发行版及每个LTS版本都有详细的配置介绍：https://developer.aliyun.com/mirror/ubuntu?spm=a2c6h.13651102.0.0.3e221b11MTIJyW

   deb https://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
   deb-src https://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
   
   deb https://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
   deb-src https://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
   
   deb https://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
   deb-src https://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
   
   # deb https://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
   # deb-src https://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
   
   deb https://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
   deb-src https://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse

2. occur error when use apt update:

   Err https://mirrors.aliyun.com/docker-ce/linux/ubuntu focal InRelease                                                                                                                                     
     The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8                                                                                       
   Fetched 0 B in 0s (0 B/s)                                                                                                                                                                                 
   
   Error during update 
   
   A problem occurred during the update. This is usually some sort of 
   network problem, please check your network connection and retry.

FIX:先将docker的这个重命名，以后再说