调试记录-docker获取镜像失败

问题描述:
使用docker pull ubuntu命令获取镜像失败，具体信息如下：

vmuser@ubuntu:/home$ docker pull ubuntu
Using default tag: latest
latest: Pulling from library/ubuntu
a4a2a29f9ba4: Pulling fs layer 
127c9761dcba: Pulling fs layer 
d13bf203e905: Pulling fs layer 
4039240d2e0b: Waiting 
error pulling image configuration: Get https://production.cloudflare.docker.com/
registry-/docker/registry/v2/blobs/sha256/74/74435f89ab7825e19cf8c92c7b5c
5ebd73ae2d0a2be16f49b3fb81c9062ab303/data?verify=1592886037-1vGgflo
iv4QZTJJOijrpvE%2FK708%3D: dial tcp 104.18.122.25:443: i/o timeout

问题分析：
ubuntu时间不对，导致下载失败，需要更新ubuntu系统时间。时间已经和网络同步了，还是不对，因为时区选择错误，需要修改时区。
查看现在的时区

vmuser@ubuntu:/home$ date -R
Mon, 22 Jun 2020 22:29:16 -0700
#时区应该是东八区+0800，这里-0700，不对，需要修改

修改时区的命令如下：

timedatectl set-timezone Asia/Shanghai 

也可以采用比较原始的方式：

vmuser@ubuntu:/home$ tzselect
Please identify a location so that time zone rules can be set correctly.
Please select a continent, ocean, "coord", or "TZ".
 1) Africa
 2) Americas
 3) Antarctica
 4) Asia
 5) Atlantic Ocean
 6) Australia
 7) Europe
 8) Indian Ocean
 9) Pacific Ocean
10) coord - I want to use geographical coordinates.
11) TZ - I want to specify the time zone using the Posix TZ format.
#? 4
Please select a country whose clocks agree with yours.
 1) Afghanistan		  18) Israel		    35) Palestine
 2) Armenia		  19) Japan		    36) Philippines
 3) Azerbaijan		  20) Jordan		    37) Qatar
 4) Bahrain		  21) Kazakhstan	    38) Russia
 5) Bangladesh		  22) Korea (North)	    39) Saudi Arabia
 6) Bhutan		  23) Korea (South)	    40) Singapore
 7) Brunei		  24) Kuwait		    41) Sri Lanka
 8) Cambodia		  25) Kyrgyzstan	    42) Syria
 9) China		  26) Laos		    43) Taiwan
10) Cyprus		  27) Lebanon		    44) Tajikistan
11) East Timor		  28) Macau		    45) Thailand
12) Georgia		  29) Malaysia		    46) Turkmenistan
13) Hong Kong		  30) Mongolia		    47) United Arab Emirates
14) India		  31) Myanmar (Burma)	    48) Uzbekistan
15) Indonesia		  32) Nepal		    49) Vietnam
16) Iran		  33) Oman		    50) Yemen
17) Iraq		  34) Pakistan
#? 9
Please select one of the following time zone regions.
1) Beijing Time
2) Xinjiang Time
#? 1

The following information has been given:

	China
	Beijing Time

Therefore TZ='Asia/Shanghai' will be used.
Selected time is now:	Tue Jun 23 13:30:22 CST 2020.
Universal Time is now:	Tue Jun 23 05:30:22 UTC 2020.
Is the above information OK?
1) Yes
2) No
#? 1

You can make this change permanent for yourself by appending the line
	TZ='Asia/Shanghai'; export TZ
to the file '.profile' in your home directory; then log out and log in again.

Here is that TZ value again, this time on standard output so that you
can use the /usr/bin/tzselect command in shell scripts:
Asia/Shanghai

还需要如下命令，才能生效：

vmuser@ubuntu:/home$ sudo cp /usr/share/zoneinfo/Asia/Shanghai  /etc/localtime
vmuser@ubuntu:/home$ date -R
Tue, 23 Jun 2020 13:34:08 +0800
#时区已经改为东8区

再次拉取docker镜像，成功！

vmuser@ubuntu:/home$ docker pull ubuntu:18.04
18.04: Pulling from library/ubuntu
d7c3167c320d: Pull complete 
131f805ec7fd: Pull complete 
322ed380e680: Pull complete 
6ac240b13098: Pull complete 
Digest: sha256:86510528ab9cd7b64209cbbe6946e094a6d10c6db21def64a93ebdd20011de1d
Status: Downloaded newer image for ubuntu:18.04
docker.io/library/ubuntu:18.04