Cached COPY layers are expensive in that they both need to be retrieved
over the network and occupy space in the layer cache.
They are unnecessary in that we already have all resources needed to
execute the COPY locally, and doing so is a trivial file-system
operation. This is in contrast to RUN layers, which can do
arbitrary and unbounded work.
The end result is that cached COPY commands were more expensive when
cached, not less. Remove them.
Resolves#1357
1240333657