Use fewest-build-containers as concourse container strategy
Use fewest-build-containers as concourse container strategy

- This strategy prioritizes running builds over caching resources
  which may serve us better given the number of tasks we have

[#166004922]

Co-authored-by: Chhavi Kankaria <a45f933d80ef78816bcf302f872972e3887e5115@pivotal.io>

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci