Propogate the DOCKER_CONFIG environment variable

What does this MR do?

This allows us to pass a custom location/config to avoid hitting Docker Hub rate limits as experienced in gitlab-org/gitlab#283929 (closed)

What are the relevant issue numbers?

1. !37 (comment 1043299017)
2. gitlab-org/gitlab#355814 (closed)
3. #40 (moved)

cc @drewcimino @iamricecake @morefice