WFM.

Tests passed for Fedora 28:

• Install singularity and debootstrap
• Cloned a definition file repo we have
• Built our base ubuntu image as squashfs formated container ( singularity build ubuntu.simg ubuntu.def")
• Converted this squashfs container to a writable ext3 format ("singularity build --writable ubuntu.img ubuntu.simg")
• Shelled into new writable image and installed python ("singularity shell --writable ubuntu.img; apt-get -y install python-dev")
• Executed python within the container, checking version ("singularity exec ubuntu.img python --version")
• Pulled latest Docker Hub Ubuntu image into a squashfs formtted container ("singularity pull docker://ubuntu:latest")
• Checked Ubuntu version of Docker-based image with 'exec' ("singularity exec ubuntu-latest.simg cat /etc/*release")

Works for me. No regressions noted compared to previous version.