gitignore: ignore .docker-image-*

We can ignore the .docker-image-* files created by
Makefile.packaging.

Change-Id: I14f08efd51f1440749c210708f600a7f3729e065
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>

.gitignore

@@ -9,3 +9,4 @@ ring_*.tar.gz
 Makefile.packaging.distro_targets
 repositories
 manual-download
+.docker-image-*