Update Makefile to change image for rootbuilder

This will no longer require people build your rootbuilder ( https://github.com/progrium/rootbuilder ) in their host before using this busybox rootfs maker :)
Change Dockerimage to progrium/rootbuilder

same as https://github.com/paimpozhil/busybox/commit/df7497ce652517606b67b38cbc53c282353808bb
Merge pull request #3 from paimpozhil/master

Update Makefile to change image for rootbuilder