Do not hardcoded username and group

[mamchenkov]

For chown/chgrp operations we need to know which username and
group to use. This varies from system to system and is not
always reliable.

Therefor we use the $USER environment variable, assuming that:

1. The variable is defined and set to the current username.
2. The group with the same name exists
3. The current user is the member of that group

This works well for combinatinos like root:root, nginx:nginx,
someuser:someuser, etc.

[georgeconstantinou]

👍