unification of image specification in docker compose files

[MajoBerger]

somewhere we use:
"${DOCKER_OWNER:-dataquest}/dspace-cli:${DSPACE_VER:-dspace-7_x}"
elsewhere we use
${DSPACE_CI_IMAGE:-dataquest/dspace:dspace-7_x-test}

It would be better to unify it, so that it is simpler and more straightforward to use. But this is spread across all the docker compose files and is not a high priority right now.