Related to https://gitlab.com/gitlab-org/charts/gitlab/-/issues/2423

Move to Debian Buster vs Debian Stretch.

Impacted by https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=962535

Impacting customers in the move from VMs (Ubuntu) to Debian Stretch in
containers of CNG.

We should consider triggering the UBI build-related jobs when the branch
name matches '-ubi8$'. Otherwise, the user has to manually trigger a
pipeline with `UBI_PIPELINE=true` every time.

Adds more detail to the UBI images documentation, including context
regarding why they are built and how to build them using separate
Dockerfiles. Also includes a note on the pipeline conditions required
for the images to be built and pushed.

* By default, should be an empty value and external dependencies are
  used.

* When set, it will be used for building alpine-certificates, cfssl-self-sign,
  gitlab-container-registry, gitlab-python, kubectl and postgresql container images

* Added to sync-images job

* Document the build time link between images based on their FROM
  requirements

Make further changes to ensure that we're not stating unicorn, where we
should be saying webservice, but also change from refernces to `Rails`
for documentation / description purposes.

We no longer use this image in the charts as of GitLab 12.7 / Chart
3.0.0

This commit also includes improvements to offline build scripts to
allow them to be executed from any location.

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