Cache gems in docker images

Dockerfile

@@ -13,15 +13,16 @@ RUN rm -rf /etc/service/sshd /etc/my_init.d/00_regen_ssh_host_keys.sh
 RUN apt-get update
 RUN apt-get install sqlite3 libsqlite3-dev
 
-# Copy in app and config files
-ADD nginx/rails-env.conf /etc/nginx/main.d/rails-env.conf
-ADD nginx/webapp.conf /etc/nginx/sites-enabled/webapp.conf
-ADD . /home/app/webapp
-
-# Install gems
+# Install gems--do this early to avoid cache invalidation
+ADD Gemfile* /home/app/webapp/
 WORKDIR /home/app/webapp
 RUN bundle install
 
+# Copy in app and config files
+ADD nginx/rails-env.conf /etc/nginx/main.d/
+ADD nginx/webapp.conf /etc/nginx/sites-enabled/
+ADD . /home/app/webapp
+
 # Install "production" database (for demo purposes only)
 WORKDIR /home/app/webapp
 RUN RAILS_ENV=production rake db:migrate