cli: show error when popper run -e singularity runs in docker (#899)
cli: show error when popper run -e singularity runs in docker (#899)

throw error message to notify users when attempting to use the singularity runner
when popper is being executed inside a docker container.

In order to identify this, the SingularityRunner constructor looks at /proc/1/cgroup
to check if docker folders are available, which is an indication that a process is 
being executed inside a docker container (https://stackoverflow.com/a/20012536)

The message instructs users to install popper via PIP instead.
systemslab/popper,systemslab/popper