Run ansible configuration during docker build process

Currently, ansible configuration is run in the Docker run phase. This has a couple of disadvantages:

1. Startup time, see https://github.com/splunk/docker-splunk/issues/10
2. Existing etc files need to be converted to YAML files, see Create Custom Configs
3. Uncommon for Docker - build phase is usually used for configuration

My expectation would be that I could do the ansible configuration first during build time and then just run the splunk process with Docker run, e.g. like this:

FROM splunk/splunk:8.2.4

ENV SPLUNK_START_ARGS --accept-license
ENV SPLUNK_PASSWORD splunkdev

# Configure using ansible
USER ansible
COPY ./tmp /tmp
RUN /sbin/entrypoint.sh start-and-exit

USER splunk

# Do whatever post-configuration, change etc files, copy apps etc.

ENTRYPOINT ["/opt/splunk/bin/splunk"]
CMD ["start"]

Start with /sbin/entrypoint.sh and the Dockerfile flow shown in the issue, comparing how Ansible configuration currently runs during Docker startup. The work is done when configuration can be applied during the image build and the runtime entrypoint only starts Splunk, while the documented custom configuration workflow remains usable.