Use Dockerfile flow for QA Auto DevOps

This results in 40MB images vs 400MB images which is faster to upload
and download. This does mean we lose the default herokuish QA coverage
but we should have adequate coverage in
https://gitlab.com/gitlab-org/cluster-integration/auto-build-image/ now.

Triva: herokuish is bigger because it's base image is based on `ubuntu`.

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"]