Update node.js and imagemagick (mastodon#1951)

* update Dockerfile: latest nodejs LTS

* also update imagemagick

Dockerfile

@@ -12,20 +12,22 @@ WORKDIR /mastodon
 
 COPY Gemfile Gemfile.lock package.json yarn.lock /mastodon/
 
-RUN BUILD_DEPS=" \
+RUN echo "@edge https://nl.alpinelinux.org/alpine/edge/main" >> /etc/apk/repositories \
+ && BUILD_DEPS=" \
  postgresql-dev \
  libxml2-dev \
  libxslt-dev \
  build-base" \
  && apk -U upgrade && apk add \
  $BUILD_DEPS \
- nodejs \
+ nodejs@edge \
+ nodejs-npm@edge \
  libpq \
  libxml2 \
  libxslt \
  ffmpeg \
  file \
- imagemagick \
+ imagemagick@edge \
  && npm install -g npm@3 && npm install -g yarn \
  && bundle install --deployment --without test development \
  && yarn --ignore-optional \