Upgrade Node.js to v8.x on Docker image (#8019)

Dockerfile

@@ -1,3 +1,4 @@
+FROM node:8.11.3-alpine as node
 FROM ruby:2.4.4-alpine3.6
 
 LABEL maintainer="https://github.com/tootsuite/mastodon" \
@@ -11,8 +12,6 @@ ENV PATH=/mastodon/bin:$PATH \
     RAILS_ENV=production \
     NODE_ENV=production
 
-ARG YARN_VERSION=1.3.2
-ARG YARN_DOWNLOAD_SHA256=6cfe82e530ef0837212f13e45c1565ba53f5199eec2527b85ecbcd88bf26821d
 ARG LIBICONV_VERSION=1.15
 ARG LIBICONV_DOWNLOAD_SHA256=ccf536620a45458d26ba83887a983b96827001e92a13847b45e4925cc8913178
 
@@ -20,6 +19,11 @@ EXPOSE 3000 4000
 
 WORKDIR /mastodon
 
+COPY --from=node /usr/local/bin/node /usr/local/bin/node
+COPY --from=node /usr/local/lib/node_modules /usr/local/lib/node_modules
+COPY --from=node /usr/local/bin/npm /usr/local/bin/npm
+COPY --from=node /opt/yarn-* /opt/yarn
+
 RUN apk -U upgrade \
  && apk add -t build-dependencies \
     build-base \
@@ -39,20 +43,13 @@ RUN apk -U upgrade \
     imagemagick \
     libidn \
     libpq \
-    nodejs \
-    nodejs-npm \
     protobuf \
     tini \
     tzdata \
  && update-ca-certificates \
- && mkdir -p /tmp/src /opt \
- && wget -O yarn.tar.gz "https://github.com/yarnpkg/yarn/releases/download/v$YARN_VERSION/yarn-v$YARN_VERSION.tar.gz" \
- && echo "$YARN_DOWNLOAD_SHA256 *yarn.tar.gz" | sha256sum -c - \
- && tar -xzf yarn.tar.gz -C /tmp/src \
- && rm yarn.tar.gz \
- && mv /tmp/src/yarn-v$YARN_VERSION /opt/yarn \
  && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarn \
  && ln -s /opt/yarn/bin/yarnpkg /usr/local/bin/yarnpkg \
+ && mkdir -p /tmp/src /opt \
  && wget -O libiconv.tar.gz "https://ftp.gnu.org/pub/gnu/libiconv/libiconv-$LIBICONV_VERSION.tar.gz" \
  && echo "$LIBICONV_DOWNLOAD_SHA256 *libiconv.tar.gz" | sha256sum -c - \
  && tar -xzf libiconv.tar.gz -C /tmp/src \