Merge branch 'faster_qa_auto_devops' into 'master'

Use Dockerfile flow for QA Auto DevOps

See merge request gitlab-org/gitlab-ce!28648

qa/qa/fixtures/auto_devops_rack/Dockerfile

+FROM ruby:2.6.3-alpine
+ADD ./ /app/
+WORKDIR /app
+ENV RACK_ENV production
+ENV PORT 5000
+EXPOSE 5000
+
+RUN bundle install
+CMD ["bundle","exec", "rackup", "-p", "5000"]