Re-enable CI variable forwarding

This is a re-roll of !22 (merged), this time accounting for the fact that not all registries are yet compatible with Docker BuildKit, by conditionally enabling that experimental feature only when AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES is requested.

See #12 (closed)

Original issue https://gitlab.com/gitlab-org/gitlab-ce/issues/54681

Manual QA performed

On top of adding a regression test for the redhat registry, I repeated the steps from !22 (merged):

1. Updated my test project on GitLab.com to use the image from this MR, registry.gitlab.com/gitlab-org/cluster-integration/auto-build-image/re-add-auto-devops-build-secrets:stable
2. Observed in the build output that the forwarded variables are present:

   #9 14.13 --------------------------------------------------------------------------------
   #9 14.13        AUTO_DEVOPS_BUILD_IMAGE_FORWARDED_CI_VARIABLES
   #9 14.13 --------------------------------------------------------------------------------
   #9 14.13        MY_VAR="It's alive!"
   #9 14.13        MY_VAR_WITH_SPECIAL_CHARACTERS="This value # is full\nof special charactrs $ !! Yay\n"
   #9 14.13 --------------------------------------------------------------------------------

See hfyngvason/test-auto-build-image-secrets for details