index.rst: Fix/add descriptions in index.rst and bad docstring

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests.docker_cli: Remove build_paths test

This test is superseded by docker_cli/build test, which now supports
various locations too plus it does deeper postprocess.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>