Improvements: GitLab local registry & ARM64 builds

This MR provides a fix for two issues: #19 (saves the container in the local GitLab container registry as well as in Docker Hub) and #28 (closed) (provides ARM architecture builds).

One important change is the renaming of the CI_REGISTRY_IMAGE variable into DOCKER_REGISTRY_IMAGE; this is because GitLab automatically sets CI_REGISTRY_IMAGE to the local URL of the container registry of the current project.