build-unit-test-docker: Customize image at the end
build-unit-test-docker: Customize image at the end

We no longer rely on the depcache to determine when to rebuild our
packages since they are now built based on the sha hash. We would delete
this but it is still used by some of the unit test scripting for
determining what dependencies our environment already has.

Also, lets do some of the cheap customizations at the end so that
intermediate containers can be more readily re-used.

Change-Id: Ibc9ae9239d693490db85bd1fb78fd8e72f75bde3
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts