Add yarn to sts4-build-env

concourse/docker/Dockerfile

@@ -15,6 +15,11 @@ RUN apt-get update && apt-get install -y \
 RUN curl -sL https://deb.nodesource.com/setup_8.x | bash - \
   && apt-get install -y nodejs
 
+RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - \
+ && echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list \
+ && apt-get update \
+ && apt-get install -y yarn
+
 RUN npm install -g vsce
 
 CMD /bin/bash