drm_hwcomposer: CI: use docker image from local container registry

drm_hwcomposer provides a .ci/Dockerfile which can be used as a base image to rebuild drm_hwcomposer. Unfortunetly, the CI stages (build, tidy, style) do not use this container image. Instead, it re-installs all the dependencies for each stage.

To use this container image, we should also build it via the CI.

Add a new build stage to build the container and publish it to the local (gitlab-provided) docker registry.

This way, it can be reused by the other build stages.

Note: as documented in [1], this requires that the configured Gitlab runner can run docker in privileged mode.

[1] https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#use-docker-in-docker

This relates a bit to #81 as it's a pre-requisite