Refactor Dockerfile to use less memory.
Refactor Dockerfile to use less memory.

Before this refactor, the docker build process had a peak memory use of
between 1.5 and 1.75 Gb.  Afterwards, the build process peaks at less
than 0.5 Gb.  Dockerhub's build is limited in memory, so this should
allow it to build there.

DigitalSlideArchive/HistomicsTK,DigitalSlideArchive/HistomicsTK