Add a "writeable home directory" to the build containers.
Each container gets its own isolated emptyDir volume mounted at "/writeable-home", to remove writes to the container's rootfs.
Should be isolated volumes since when step 1 runs as user A but step 2 as user B, there are permission issues if this directory is shared. For Git SSH, it would actually mean that we put a private key on disk which is then unnecessarily visible also for the image processing step at least.