USER is picked as the user specified by the Docker config
or if that is empty is defaulted to root. If the USER is root
then HOME is defaulted to /root otherwise it is set to the
HOME found when running `env` in a sample of the container.

PATH is set to a normal sane default.

Any of these environment variables can be overridden in the
Dockerfile or using profile parameters at any time. This is
just to help users who rely on Docker providing defaults.