docker: Only attempt to pull each image once
docker: Only attempt to pull each image once

Pulling an image takes quite a while, even if it's already been
pulled.  This patch shaves off a lot of time by simply remembering
which images have already been pulled so that they only need to be
pulled once.

kayousterhout/quilt,kelda/kelda,kayousterhout/quilt,quilt/quilt,kklin/quilt,NetSys/quilt,mmussomele/quilt,kklin/kelda,kelda/kelda,stevenlin598/quilt,quilt/quilt,ejj/quilt-dev,PenguinToast/quilt,akshayknarayan/quilt,tejasmanohar/quilt,kklin/kelda,LuiseV/quilt,PenguinToast/quilt,NetSys/quilt,LuiseV/quilt,quilt/quilt,ejj/quilt-dev,mmussomele/quilt,stevenlin598/quilt,PenguinToast/quilt,tejasmanohar/quilt,kayousterhout/quilt,stevenlin598/quilt,secant/quilt,kklin/quilt,kklin/quilt,ZhongtianWang/quilt,kklin/quilt,kayousterhout/quilt,stevenlin598/quilt,tejasmanohar/quilt,stevenlin598/quilt,ZhongtianWang/quilt,kelda/kelda,ejj/quilt-dev,mmussomele/quilt,secant/quilt,akshayknarayan/quilt,PenguinToast/quilt,secant/quilt,stevenlin598/quilt,LuiseV/quilt,ejj/quilt-dev,NetSys/quilt,akshayknarayan/quilt,ZhongtianWang/quilt,PenguinToast/quilt,mmussomele/quilt,mmussomele/quilt,secant/quilt,NetSys/quilt,akshayknarayan/quilt,NetSys/quilt,kelda/kelda,LuiseV/quilt,tejasmanohar/quilt,mmussomele/quilt,kklin/kelda,LuiseV/quilt,quilt/quilt,ZhongtianWang/quilt,kklin/kelda,secant/quilt