supervisord uses the Python string expression syntax in its
configuration files, like this `%(ENV_X)s `
It's described here:
http://supervisord.org/configuration.html#environment-variables
In the "Deploy->Supervisor->Configure" section of the documentation the
final "s" were omitted:
https://docs.vapor.codes/deploy/supervisor/#configure
so copy-pasting this example was leading to wrong file names of the
logs.
For example, instead of the expected file name `hello-stdout.log` I had
the wrong `hellotdout.log`.