Better way to skip export of images during build

[Ichmed]

Because build.sh can only be executed after the initial setup, there is no image to export during the init step. This is fixed right now by skipping the execution in the builder which is not an elegant way of doing this.

A better solution probably requires restructuring the builder architecture somewhat.

Proposal: Refactor the "dry-run", "skip-export" and "keep" flags into a holistic pipeline where each individual step can be skipped via command line option --skip=STEP_NAME