Revert "Remove deprecated DOCKER_HUB_USER variables"
Revert "Remove deprecated DOCKER_HUB_USER variables"

This reverts commit fbe85162182331d0b76d2046d09ca61cd2660b24.

buildkite/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws