We now try to emulate any simple COPY <src> <dst> instructions via rsync
prior to image build.

This *does* mean that artifact builder scripts must be careful to create
all necessary dirs according to the base image semantics, because the
base image content is not there when we emulate the COPY instructions.
For instance, many of the modified Dockerfile-runit and
runit-artifacts.sh files depended on built runit packages being
installed into /tmp in the final image -- but they didn't create the
/tmp dir because the COPY instruction they used was running atop a
fully-populated base image that already had /tmp.  Thus, the
runit-artifacts.sh scripts had to be changed to create /tmp with the
proper permissions.