Clean yarn cache in prod image build

Actually this was suggested by some Dockerfile lint display
integrated with neovim.

Author: simonbaird

docker/Dockerfile.prod

@@ -26,7 +26,7 @@ RUN bundle install
 
 # Install node modules
 COPY --chown=$APP_USER:$APP_USER rails/package.json rails/yarn.lock ./
-RUN yarn install --production
+RUN yarn install --production && yarn cache clean
 
 # Install the rails app, including some static files placed under rails/public,
 # excluding what's listed in .dockerignore