Use global settings for busybox image
Summary
Similar to !864 (merged)
We should move our use of the busybox image to a global, in order to make it easy to switch to another image. This is especially useful for install on isolated networks where users are bringing their own images: #1613 and was brought up as part of a distribution engagement call: https://gitlab.com/gitlab-org/distribution/team-tasks/issues/512