Allow to set GROUPNAME and avoid inferring it

[felipecrs]

This partially reverts e96e695, as I don't think it was a good idea.

But now, the user can pass a different groupname with GROUPNAME environment variable, like:

ARG USERNAME=username
ARG GROUPNAME=groupname
RUN <fixdockergid>/install.sh

While still falling back to USERNAME if GROUPNAME is not set.

Also, to avoid issues in runtime, we verify the presence of the user and group during install.sh execution.