I’m afraid that our builder can’t pull from private repositories at the moment, since it would somehow have to be authenticated against your private repository.
As discussed in this forum thread:
a workaround would be to use resin build and resin deploy locally in order to use you private images.
On the other hand, if the compilation step is independent of the previous steps, you could build a minimal public base image for that and then use multistaged builds, have an extra FROM referencing that base image and only copy the built artifacts. Let me point you to the respective documentation page: