Merge pull request #2152 from ronen/patch-1

Bug fix: Use app's Gemfile.lock in Dockerfile

docs/rails.md

@@ -26,6 +26,7 @@ Dockerfile consists of:
     RUN mkdir /myapp
     WORKDIR /myapp
     ADD Gemfile /myapp/Gemfile
+    ADD Gemfile.lock /myapp/Gemfile.lock
     RUN bundle install
     ADD . /myapp