Add COPY .ruby-version to Dockerfile build

Author: viniciusoyama

Dockerfile

@@ -27,7 +27,7 @@ RUN apt-get update -qq && \
 RUN npm install -g yarn
 
 # Install application gems
-COPY Gemfile Gemfile.lock ./
+COPY Gemfile Gemfile.lock .ruby-version ./
 RUN bundle install && \
     rm -rf ~/.bundle/ "${BUNDLE_PATH}"/ruby/*/cache "${BUNDLE_PATH}"/ruby/*/bundler/gems/*/.git