source-to-image: Incremental build error (read/write on closed pipe)

Saving incremental artifacts seems to fail randomly on (perhaps just large) projects. Could be a regression of #171, although ulimit and filesystem hard limits doesn’t seem to affect it.

I’ve created a deployable project to replicate the issue, available at:

https://github.com/binarybabel/sti-rails-incremental-demo Build config and logs are available in the extra directory.

Level 5 build log illustrating the problem:

https://raw.githubusercontent.com/binarybabel/sti-rails-incremental-demo/master/extra/incremental-failure.log