Apply DSOP Dockerfile guidelines on UBI-based CNG images

Closes gitlab-org/distribution/team-tasks#505

See merge request gitlab-org/build/CNG!294

Update CNG release process docs

See merge request gitlab-org/build/CNG!296

Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

Bumps mailroom to 0.9.1

See merge request gitlab-org/build/CNG!295

backup-utility: s3cmd improvements

See merge request gitlab-org/build/CNG!289

Use master as the default GITLAB_ELASTICSEARCH_INDEXER_VERSION

See merge request gitlab-org/build/CNG!279

Remove password deletion from UBI-based images

See merge request gitlab-org/build/CNG!293

Bump container registry to v2.7.2-gitlab

See merge request gitlab-org/build/CNG!292

This brings in the MD5 checksum changes in
https://gitlab.com/gitlab-org/container-registry/merge_requests/1.

Push tags based on container registry version

See merge request gitlab-org/build/CNG!291

Only pull the container registry from .com

See merge request gitlab-org/build/CNG!290

For now, we decided to only get the container registry from .com, just
as an initial step, until security releases from dev are supported for
the registry.

Add images for container registry (UBI and Debian-based)

Closes gitlab-org/distribution/team-tasks#499

See merge request gitlab-org/build/CNG!287

We switched to Debian from Alpine because Alpine yanks non-latest
packages from their repos.  This impacts our ability to pin packages,
match Omnibus, and release backports.

Move binary to /bin/registry and use default config of
/etc/docker/registry/config.yml.

Remove unnecessary scripts

Use Alpine image to get container smaller

TAG -> GIT_TAG in .gitlab-ci.yml

Properly tag Go image

Remove EE-specific limitation in .gitlab-ci.yml