Docker image build for security release

This is a spin off from https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1679

When we do a security release, we now have to do a manual build because our docker image is being built from a public repository. When PACKAGECLOUD_REPO is set as an env variable, at the moment this is an indication that we are doing a security release

We need to:

• Be able to pass the private repository to our Docker build so it can find the package.
• Push that newly built image to our private registry. I suggest enabling registry for CE and EE on dev so we can have that separation which might help us with scripting the last step.

Last step at the moment would be to pull the images from the private registry and sync them to docker store. It is OK to have this as a manual step. We just need to think about how we can script this easy because this will be a part of package promote task.