Add clarification in usage about hostname used

[benoit74]

@tw4l @ikreymer this is ready for review, mostly minor change but we've had the question so better to clarify for everyone

[ikreymer]

This is actually the container name, not the machine hostname! We don't have a way to access the machine hostname, just using os.hostname() which is the hostname of the container.

[benoit74]

Indeed, I clarified it a bit further