https://skarnet.org/software/s6/
https://skarnet.org/software/s6/overview.html
https://skarnet.org/software/s6/s6-envdir.html

https://github.com/just-containers/s6-overlay

2014
https://blog.tutum.co/2014/12/02/docker-and-s6-my-new-favorite-process-supervisor/

samples
/usr/bin/withcontenv
    https://github.com/just-containers/s6-overlay#container-environment
    https://stackoverflow.com/questions/57342546/what-is-the-source-of-a-with-contenv-shebang