Add a helper script for releasing UBI source

This MR contains a helper script to facilitate the source release to https://dccscr.dsop.io/dsop/gitlab/gitlab.

Usage

USAGE:

  release-helper.sh RELEASE_TAG RELEASE_PATH PREVIOUS_RELEASE [SOURCE]

    RELEASE_TAG       GitLab release tag (e.g. v12.5.1-ubi8). The scripts creates
                      a directory for the release tag (e.g. 12.5).

    RELEASE_PATH      The release root directory. A directory for the new
                      release will be created here. The previous release 
                      directory will be looked up in here as well.

    PREVIOUS_RELEASE  The directory name of the previous release (e.g. 12.4).
                      Some assets will be copied from the previous release.

    SOURCE            The root directory of GitLab CNG repository. The current
                      directory is used when not specified.

For example:

`build-scripts/ubi-offline/release-helper.sh v12.6.0-ubi8 ./build 12.5

What does it do?

• Creates a directory for the new release using the specified release tag.
• Duplicates the UBI image directories (Dockerfiles and assets) and restructures and renames the files where needed.
• Replaces CI-specific Dockerfile arguments.
• Prepends the build stage Dockerfile instructions.
• Copies some of the assets from the previous release, including, LICENSE, README.md, and build-scripts/

Caveats

• This script does not set the version build arguments, such as REGISTRY_VERSION in gitlab-container-registry or GITALY_VERSION in gitaly.

Closes https://gitlab.com/gitlab-org/charts/gitlab/issues/1695