Linux基于alpine构建镜像时错误：temporary error (try again later)，ERROR: unsatisfiable constraints 解决方案

首先检查DNS及与请求源地址的网络连接是否正常：

curl -v -s http://dl-cdn.alpinelinux.org/alpine/edge/main/x86_64/APKINDEX.tar.gz > /dev/null

如果返回正常且构建时仍然报如下错误：

fetch http://dl-cdn.alpinelinux.org/alpine/v3.12/main/x86_64/APKINDEX.tar.gz
WARNING: Ignoring http://dl-cdn.alpinelinux.org/alpine/v3.12/main/x86_64/APKINDEX.tar.gz: temporary error (try again later)
fetch http://dl-cdn.alpinelinux.org/alpine/v3.12/community/x86_64/APKINDEX.tar.gz
WARNING: Ignoring http://dl-cdn.alpinelinux.org/alpine/v3.12/community/x86_64/APKINDEX.tar.gz: temporary error (try again later)
ERROR: unsatisfiable constraints:
.........

请尝试将防火墙后端服务改为iptables并重启防火墙：

vi /etc/firewalld/firewalld.conf

编辑文档
找到：FirewallBackend=nftables
改为：FirewallBackend=iptables

然后重启防火墙：

sudo systemctl restart firewalld.service