dockerfile 的问题 FROM alpine:3.8 temporary error (try again later)

FROM alpine:3.8

 apk add xxx安装软件 

fetch http://dl-cdn.alpinelinux.org/alpine/v3.8/main/x86_64/APKINDEX.tar.gz

WARNING: Ignoring http://dl-cdn.alpinelinux.org/alpine/v3.8/main/x86_64/APKINDEX.tar.gz: temporary error (try again later)
fetch http://dl-cdn.alpinelinux.org/alpine/v3.8/community/x86_64/APKINDEX.tar.gz

WARNING: Ignoring http://dl-cdn.alpinelinux.org/alpine/v3.8/community/x86_64/APKINDEX.tar.gz: temporary error (try again later)
ERROR: unsatisfiable constraints:
  .persistent-deps-0:
    masked in: cache
    satisfies: world[.persistent-deps]
  ca-certificates (missing):
    required by:
  curl (missing):
    required by:
  tar (missing):
    required by:
  xz (missing):
    required by:
  libressl (missing):
    required by:
The command '/bin/sh -c apk add --no-cache --virtual .persistent-deps                 ca-certificates                 curl                 tar                 xz                 libressl' returned a non-zero code: 6

解决：

#vim /etc/default/docker

#DOCKER_OPTS="--dns 114.114.114.114"

#systemctl restart docker