MINOR: debug: report in post_mortem if the container techno used is docker

MINOR: debug: report in post_mortem if the container techno used is docker

If we detect we're running inside a container on Linux, let's check if
it seems to be docker. Docker usually creates a /.dockerenv file, which
is easy to check. It's uncertain whether it's always the case, but on the
few tested instances that was true, and we don't really care, what matters
is to place helpful debugging info for developers. When this file is
detected, we report "docker" instead of "yes" in the container techno.