DEV: Clean yarn cache after yarn install...

DEV: Clean yarn cache after yarn install (https://github.com/discourse/discourse_docker/commit/b259c8d38e0f42288fd279c9f9efd3cefbc2c1cb)

Dockerfile

@@ -111,6 +111,8 @@ RUN \
 
 ### Gem installation
 RUN exec bundle install --deployment --jobs 4 --without test --without development && \
+    # Do not install yarn as root, it comes from the base image.
+    yarn cache clean && \
     exec bundle exec rake maxminddb:get && \
     find /discourse/vendor/bundle -name tmp -type d -exec rm -rf {} +