ci: Makefile: Expose the CI_USER_LOGIN variable for users to use

ci: Makefile: Expose the CI_USER_LOGIN variable for users to use

More often than not I find myself debugging in the containers which
means that I need to have root inside, but without manually tweaking
the Makefile each time the execution would simply fail thanks to the
uid/gid mapping we do. What if we expose the CI_USER_LOGIN variable, so
that when needed, the root can be simply passed with this variable and
voila - you have a root shell inside the container with CWD=~root.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>