When doing an incremental build, the s2i builder pulls its builder image before calling the save-artifacts script and doesn't ensure that the builder image is still there when it calls assemble.
This leaves a gap of time between the start of the build and the calling of the assemble script in which the image can be removed.
If the image is removed, the build fails.
Add a call to ensure that the builder image exists right before calling the assemble script.
The chance of the assemble script running and not finding an available builder image is greatly reduced.