Use docker import/export to make image smaller

Using docker export gives us only the top filesystem layer and not all
the layers used to build. Resulting image is about 20% smaller. Further
optimizations will be made in future commits via removing build time
deps and temporary files in the Dockerfile.

Change-Id: I0f7ad6d00c6f47836dcd71b05907076701bb2789
(cherry picked from commit 99dba22eaa3961234abde1294e55026fd2cc9d4d)