Fix dead links for Docker Build (CE)

What does this MR do?

Correct links for Docker Build under:

• https://docs.gitlab.com/ce/user/project/container_registry.html#build-and-push-images-using-gitlab-ci

The First two links are 404

• Using Docker Build link points here (https://docs.gitlab.com/ce/user/ci/docker/using_docker_build.html)
• Using the GitLab Container Registry documentation link points here (https://docs.gitlab.com/ce/user/ci/docker/using_docker_build.html#using-the-gitlab-container-registry)

Correct links should not include /user/

\cc @connorshea