Support usage of docker container proxy for buildah image download

This feature request targets Gitlab self-hosted deployment.

In some restrictive environments, pulling docker images from the Internet directly is highly discouraged or even blocked. Therefore, it would be great that the buildah docker image reference used in the .gitlab-ci.yml file could be reworked to use a CI variable that we can overwrite at the project level (of the project mirror), to force the buildah image download to go through a docker container pull through proxy cache (Nexus, Artifactory, etc).

This way, we can mirror the Opentofu component, then build release on the self-hosted instance without pulling docker images directly.