shrink image, install via yarn, fix arm build

Dockerfile.armhf

@@ -18,6 +18,9 @@ RUN \
  curl -s https://deb.nodesource.com/gpgkey/nodesource.gpg.key | apt-key add - && \
  echo 'deb https://deb.nodesource.com/node_12.x bionic main' \
 	> /etc/apt/sources.list.d/nodesource.list && \
+ curl -s https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - && \
+ echo 'deb https://dl.yarnpkg.com/debian/ stable main' \
+	> /etc/apt/sources.list.d/yarn.list && \
  echo "**** install build dependencies ****" && \
  apt-get update && \
  apt-get install -y \
@@ -33,14 +36,17 @@ RUN \
 	nano \
 	net-tools \
 	nodejs \
-	sudo && \
+	sudo \
+	yarn && \
  echo "**** install code-server ****" && \
  if [ -z ${CODE_RELEASE+x} ]; then \
 	CODE_RELEASE=$(curl -sX GET "https://api.github.com/repos/cdr/code-server/releases/latest" \
 	| awk '/tag_name/{print $4;exit}' FS='[""]'); \
  fi && \
  CODE_VERSION=$(echo "$CODE_RELEASE" | awk '{print substr($1,2); }') && \
- npm install --unsafe-perm -g code-server@"$CODE_VERSION" && \
+ yarn --production global add code-server@"$CODE_VERSION" && \
+ yarn cache clean && \
+ ln -s /node_modules/.bin/code-server /usr/bin/code-server && \
  echo "**** clean up ****" && \
  apt-get purge --auto-remove -y \
 	build-essential \