Preserve /build/{cleanup.sh,buildconfig} files.
Preserve /build/{cleanup.sh,buildconfig} files.

These files are useful for those who create Docker images based on this
baseimage, and need to make a final cleanup and/or use some
configurations that are inside buildconfig file.

StepsAway/baseimage-docker,remeeting/baseimage-docker,phusion/baseimage-docker,remeeting/baseimage-docker,jedisct1/phusion-baseimage-latest,StepsAway/baseimage-docker,jedisct1/phusion-baseimage-latest,REDDOXX/baseimage-docker,phusion/baseimage-docker,REDDOXX/baseimage-docker