Ubuntu版本：19.10
按照docker官网的安装方法，在Ubuntu上按装docker时报错：
Package containerd.io 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 'containerd.io' has no installation candidate
原因是执行命令
$ sudo add-apt-repository \
"deb [arch=amd64] https://download.docker.com/linux/ubuntu \
$(lsb_release -cs) \
stable"
其中：$(lsb_release -cs)返回的“eoan”即Ubuntu 19.10。猜测可能是Ubuntu 19.10的问题，替换为表示ubuntu 18.10的“bionic”，如下：
$ sudo add-apt-repository \
"deb [arch=amd64] https://download.docker.com/linux/ubuntu \
bionic \
stable"
为了避免冲突，记得修改“/etc/apt/sources.list”，注释掉
deb-src [arch=amd64] https://download.docker.com/linux/ubuntu eoan stable