Dockerfiles use build container pattern

This change modifies the dockerfiles/build.sh script to always look for
a Dockerfile.build file from which a container will be created. The
container created from Dockerfile.build will have everything from
/tmp/cache copied into the dockerfile directory where the CI image is
created.

This pattern allows Dockerfile.build files that cache a repo and all
testing dependencies within the container itself.

Bug: T166888
Change-Id: I6b72d194e9eb353bff64c27a7308d3a6d5552bd6