我正在使用基于 debian buster 的 docker 镜像。我想安装这个包：
https://packages.debian.org/buster/php-memcache
但是当我尝试时它说未找到包：
# apt install php-memcache
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package php-memcache 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 'php-memcache' has no installation candidate
我似乎有正确的存储库：
#grep ^[^#] /etc/apt/sources.list /etc/apt/sources.list.d/*
/etc/apt/sources.list:deb http://deb.debian.org/debian buster main
/etc/apt/sources.list:deb http://security.debian.org/debian-security buster/updates main
/etc/apt/sources.list:deb http://deb.debian.org/debian buster-updates main
为什么这个包不可用？