At the layering sync meeting on Thursday, August 10th, it was decided that for this to be considered ready for Dev / Tech Preview, cluster admins need a way to inject custom Dockerfiles into their on-cluster builds.

(Commentary: It was also decided 4 months ago that this was not an MVP requirement in https://docs.google.com/document/d/1QSsq0mCgOSUoKZ2TpCWjzrQpKfMUL9thUFBMaPxYSLY/edit#heading=h.jqagm7kwv0lg. And quite frankly, this requirement should have been known at that point in time as opposed to the week before tech preview.)