Use Alpine Linux yarn package in Docker (#6725)

Yarn was manually installed to meet the Yarn version requirement of
webpacker. Today, Alpine Linux 3.7 provides Yarn new enough.

Dockerfile

@@ -9,8 +9,6 @@ ARG GID=991
 ENV RAILS_SERVE_STATIC_FILES=true \
     RAILS_ENV=production NODE_ENV=production
 
-ARG YARN_VERSION=1.5.1
-ARG YARN_DOWNLOAD_SHA256=cd31657232cf48d57fdbff55f38bfa058d2fb4950450bd34af72dac796af4de1
 ARG LIBICONV_VERSION=1.15
 ARG LIBICONV_DOWNLOAD_SHA256=ccf536620a45458d26ba83887a983b96827001e92a13847b45e4925cc8913178
 
@@ -40,14 +38,9 @@ RUN apk -U upgrade \
     protobuf \
     tini \
     tzdata \
+    yarn \
  && update-ca-certificates \
- && mkdir -p /tmp/src /opt \
+ && mkdir -p /tmp/src \
- && 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 \
  && 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 \