Add Gomplate to gitlab-pages image

What does this MR do?

This MR inserts Gomplate in gitlab-pages image.

Related issues

Part of #2694

Test

IMAGE=gitlab-pages
TAG=2694-add-gomplate-to-gitlab-pages
docker run --rm registry.gitlab.com/gitlab-org/build/cng/$IMAGE:$TAG gomplate -h
docker run --rm registry.gitlab.com/gitlab-org/build/cng/$IMAGE:$TAG-ubi8 gomplate -h

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

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