Build job optimization based on build cache #633

This is part of a set of phases outlined by #186.

This phase would optimize the build for cases where a particular build job would end up not producing any new images because all the images it would have been result in cache hits. To do this, the same logic that the build command uses to determine whether an image needs to be built would be executed as part of the build matrix generation. When that logic is run over the entire set of Dockerfiles, any build legs that do not have any Dockerfiles to be built can simply be trimmed from the matrix. This avoids executing build jobs that would end up not building any new images. This is only intended as an optimization for trimming out such jobs. The build command would still be responsible for calculating which images need to be built on its own.