The previous configuration of building with Docker used a lot of
'magic' and required a fair bit of traversal between the shell scripts
to understand what was happening and where. Most of it could be solved
by using Docker Multi-staged Builds but to not constantly break the
cache when just dealing with final artifacts I've split out the builder
Dockerfile from the main Dockerfile. I've also broken out the commands
run into individual make commands while trying to keep the original
shell script API mostly intact.

Running `./scripts/create_docker_image.sh` will work as it originally
did just with the modifications necessary to align the script to also
use the same underlying commands that are in make. I would have removed
the script completely and adjusted the build pipelines, but the Makefile
has an initial requirement of having `opam` installed which might not be
the case for all users or the base build system.

Othewise you can now just `make docker-image` but this will use the make
bindings. It might be a good idea in the future to make your own Docker
"building" image that can just use make within GitLabs CI system, but I
think that was out of scope of this change.