In docker image emulabization, attempt to combine COPY instructions.
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.
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