E: Unable to locate the package problem when installing docker under Ubuntu

Switch to the /etc/apt/ directory and execute

cd /etc/apt/

The first step is to back up the official default source

　　Avoid operating mistakes by your own hands, reinstalling the system is too time-consuming

cp /etc/apt/sources.list /etc/apt/sources.list.bak        #此命令是备份该官方源文件

Ubuntu 14.04.4 official default update source sources.list

# deb cdrom:[Ubuntu 14.04.4 LTS _Trusty Tahr_ - Release amd64 (20160217.1)]/ trusty main restricted
 
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://cn.archive.ubuntu.com/ubuntu/ trusty main restricted
deb-src http://cn.archive.ubuntu.com/ubuntu/ trusty main restricted
 
## Major bug fix updates produced after the final release of the
## distribution.
deb http://cn.archive.ubuntu.com/ubuntu/ trusty-updates main restricted
deb-src http://cn.archive.ubuntu.com/ubuntu/ trusty-updates main restricted
 
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://cn.archive.ubuntu.com/ubuntu/ trusty universe
deb-src http://cn.archive.ubuntu.com/ubuntu/ trusty universe
deb http://cn.archive.ubuntu.com/ubuntu/ trusty-updates universe
deb-src http://cn.archive.ubuntu.com/ubuntu/ trusty-updates universe
 
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://cn.archive.ubuntu.com/ubuntu/ trusty multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ trusty multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ trusty-updates multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ trusty-updates multiverse
 
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://cn.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
 
deb http://security.ubuntu.com/ubuntu trusty-security main restricted
deb-src http://security.ubuntu.com/ubuntu trusty-security main restricted
deb http://security.ubuntu.com/ubuntu trusty-security universe
deb-src http://security.ubuntu.com/ubuntu trusty-security universe
deb http://security.ubuntu.com/ubuntu trusty-security multiverse
deb-src http://security.ubuntu.com/ubuntu trusty-security multiverse
 
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu trusty partner
# deb-src http://archive.canonical.com/ubuntu trusty partner
 
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu trusty main
deb-src http://extras.ubuntu.com/ubuntu trusty main

The second step is to modify the official source to a third-party source
```
sudo vim /etc/apt/sources.list 
```
A third-party source recommended by most people on the Internet is attached:
Tsinghua source ubuntu Tsinghua source
Aliyun ubuntu Tsinghua source

Add Ali GPG key

curl -fsSL http://mirrors.aliyun.com/docker-ce/linux/ubuntu/gpg | sudo apt-key add -

if appear

root@0001:/etc/apt# sudo apt update
命中:1 http://mirrors.aliyun.com/docker-ce/linux/ubuntu disco InRelease
忽略:2 https://mirrors.aliyun.com/ubuntu trusty InRelease                                              
命中:3 https://mirrors.aliyun.com/ubuntu trusty-security InRelease       
命中:4 https://mirrors.aliyun.com/ubuntu trusty-updates InRelease        
命中:5 https://mirrors.aliyun.com/ubuntu trusty-backports InRelease                                    
命中:6 https://mirrors.aliyun.com/ubuntu trusty Release
命中:8 http://ppa.launchpad.net/linuxuprising/libpng12/ubuntu disco InRelease
正在读取软件包列表... 完成
正在分析软件包的依赖关系树       
正在读取状态信息... 完成       
有 2 个软件包可以升级。请执行 ‘apt list --upgradable’ 来查看它们。

   89  sudo add-apt-repository ppa:linuxuprising/libpng12
   90  sudo apt update
   91  sudo apt install libpng12-0
   92  sudo apt install -f
   93  sudo apt-get update
   94  curl -fsSL https://get.docker.com -o get-docker.sh
   95  DRY_RUN=1 sh ./get-docker.sh
   96  systemctl start docker
   97  docker search nginx

Then according to the docker official documentation

Install using a convenience script

 curl -fsSL https://get.docker.com -o get-docker.sh
 DRY_RUN=1 sh ./get-docker.sh

E: Unable to locate the package problem when installing docker under Ubuntu

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