Mastodonでgit pullしたらビルドエラー

# 概要
EC2で稼働させている[Mastodon](https://github.com/tootsuite/mastodon)で`git pull`した後に`docker-compose build`したら下記エラーが出てハマった。

# 環境
- AWS EC2(t2.medium)
- Ubuntu Server 16.04 LTS

# エラー内容
```
ubuntu@ip-***-***-**-**:~/mastodon$ docker-compose build
redis uses an image, skipping
db uses an image, skipping
Building streaming
Step 1/9 : FROM ruby:2.4.1-alpine
 ---> 5eadd5d1419a
Step 2/9 : LABEL maintainer "https://github.com/tootsuite/mastodon" description "A GNU Social-compatible microblogging server"
 ---> Using cache
 ---> 95a4b711ef32
Step 3/9 : ENV RAILS_ENV production NODE_ENV production
 ---> Using cache
 ---> 499e95f00e13
Step 4/9 : EXPOSE 3000 4000
 ---> Using cache
 ---> 167a91f421f4
Step 5/9 : WORKDIR /mastodon
 ---> Using cache
 ---> e185ae07f027
Step 6/9 : COPY Gemfile Gemfile.lock package.json yarn.lock /mastodon/
 ---> Using cache
 ---> 460d49537428
Step 7/9 : RUN BUILD_DEPS="     postgresql-dev     libxml2-dev     libxslt-dev     build-base"  && apk -U upgrade && apk add     $BUILD_DEPS     nodejs     libpq     libxml2     libxslt     ffmpeg     file     imagemagick  && npm install -g npm@3 && npm install -g yarn  && bundle install --deployment --without test development  && yarn --ignore-optional  && yarn cache clean  && npm -g cache clean  && apk del $BUILD_DEPS  && rm -rf /tmp/* /var/cache/apk/*
 ---> Running in 68a8d45af6e8
fetch http://dl-cdn.alpinelinux.org/alpine/v3.4/main/x86_64/APKINDEX.tar.gz
ERROR: http://dl-cdn.alpinelinux.org/alpine/v3.4/main: No such file or directory
WARNING: Ignoring APKINDEX.167438ca.tar.gz: No such file or directory
fetch http://dl-cdn.alpinelinux.org/alpine/v3.4/community/x86_64/APKINDEX.tar.gz
ERROR: http://dl-cdn.alpinelinux.org/alpine/v3.4/community: No such file or directory
WARNING: Ignoring APKINDEX.a2e6dac0.tar.gz: No such file or directory
OK: 21 MiB in 29 packages
WARNING: Ignoring APKINDEX.167438ca.tar.gz: No such file or directory
WARNING: Ignoring APKINDEX.a2e6dac0.tar.gz: No such file or directory
ERROR: unsatisfiable constraints:
  build-base (missing):
    required by: world[build-base]
  ffmpeg (missing):
    required by: world[ffmpeg]
  file (missing):
    required by: world[file]
  imagemagick (missing):
    required by: world[imagemagick]
  libpq (missing):
    required by: world[libpq]
  libxml2 (missing):
    required by: world[libxml2]
  libxml2-dev (missing):
    required by: world[libxml2-dev]
  libxslt (missing):
    required by: world[libxslt]
  libxslt-dev (missing):
    required by: world[libxslt-dev]
  nodejs (missing):
    required by: world[nodejs]
  postgresql-dev (missing):
    required by: world[postgresql-dev]
ERROR: Service 'streaming' failed to build: The command '/bin/sh -c BUILD_DEPS="     postgresql-dev     libxml2-dev     libxslt-dev     build-base"  && apk -U upgrade && apk add     $BUILD_DEPS     nodejs     libpq     libxml2     libxslt     ffmpeg     file     imagemagick  && npm install -g npm@3 && npm install -g yarn  && bundle install --deployment --without test development  && yarn --ignore-optional  && yarn cache clean  && npm -g cache clean  && apk del $BUILD_DEPS  && rm -rf /tmp/* /var/cache/apk/*' returned a non-zero code: 11
```

# 解決方法
alpinelinux.orgがダウンしているタイミングで実行していたのが悪かったみたい。
詳しくは下記 @noraworld さんのコメントを参照。