安装docker，报错：

$ sudo apt-get install docker-ce docker-ce-cli containerd.io
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Package docker-ce is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'docker-ce' has no installation candidate
E: Unable to locate package docker-ce-cli
E: Unable to locate package containerd.io

首先尝试更新，并重新安装

sudo apt-get update && apt-get upgrade
sudo apt-get install docker-ce docker-ce-cli containerd.io

如果依旧报错，说明apt的源设置的不对。

安装阿里云证书，并配置阿里云的docker源：

curl -fsSL https://mirrors.aliyun.com/docker-ce/linux/ubuntu/gpg | sudo apt-key add - #安装阿里云gpg证书
cd /etc/apt/sources.list.d        #在指定目录下新建docker.list文件，添加阿里云镜像源
sudo touch docker.list            
sudo chmod 666 docker.list
sudo echo "deb [arch=amd64] https://mirrors.aliyun.com/docker-ce/linux/ubuntu $(lsb_release -cs) stable" > docker.list

更新软件源，重新安装docker，等待安装完成：

sudo apt-get update       #更新软件源
sudo apt-get install docker-ce docker-ce-cli containerd.io    #重新安装docker