Shorten `build-qa-image` by optimizing Docker build

Problem to solve

Many of the steps in the docker build process in build-qa-image are repeated steps that can probably be cached. We are really only interested in the following steps most of the time:

WORKDIR /home/gitlab/qa
COPY ./qa/Gemfile* /home/gitlab/qa/
COPY ./config/initializers/0_inject_enterprise_edition_module.rb /home/gitlab/config/initializers/
# Copy VERSION to ensure the COPY succeeds to copy at least one file since ee/app/models/license.rb isn't present in FOSS
# The [b] part makes ./ee/app/models/license.r[b] a pattern that is allowed to return no files (which is the case in FOSS)
COPY VERSION ./ee/app/models/license.r[b] /home/gitlab/ee/app/models/
COPY ./lib/gitlab.rb /home/gitlab/lib/
COPY ./lib/gitlab/utils.rb /home/gitlab/lib/gitlab/
COPY ./INSTALLATION_TYPE ./VERSION /home/gitlab/
RUN cd /home/gitlab/qa/ && bundle install --jobs=$(nproc) --retry=3 --quiet
COPY ./qa /home/gitlab/qa

Proposal

Consider pushing an intermediary builder image and using it as cache.

/cc @gl-quality/eng-prod