Run yarn and cache packages

Run yarn and cache packages

diff --git a/image/base/Dockerfile b/image/base/Dockerfile
index 92762e1..f275930 100644
--- a/image/base/Dockerfile
+++ b/image/base/Dockerfile
@@ -140,5 +140,6 @@ RUN useradd discourse -s /bin/bash -m -U &&\
     chown -R discourse:discourse /var/www/discourse &&\
     cd /var/www/discourse &&\
     sudo -u discourse bundle install --deployment --jobs 4 --without test development &&\
+    yarn install &&\
     bundle exec rake maxminddb:get &&\
     find /var/www/discourse/vendor/bundle -name tmp -type d -exec rm -rf {} +

GitHub sha: e5419c34bbb9bba88e2ce8302f574aa6f8c5c746

This commit appears in #556 which was approved by eviltrout. It was merged by Falco.