Fix UBI-8 Rails containers not starting by default

What does this MR do?

!1541 (merged) caused CONFIG_TEMPLATE_DIRECTORY to be blank instead of the default /srv/gitlab/config.

This prevents the containers from starting without that variable set:

$ docker run -it registry.gitlab.com/gitlab-org/build/cng/gitlab-webservice-ee:master-ubi8
usage: set-config <template_directory> [<config_directory>]

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with dependencies.io