Documentation: Clarify README.md blurb on --cache-copy-layers

[korenyoni]

Relates to #1940

Description

This clarifies the blurb on --cache-copy-layers in README.md, which is misleading to readers and can imply that it controls layer caching for both RUN and COPY, but in fact only refers to COPY as the name of the flag suggests.

Related issues:

• What does the --cache-copy-layers flag do? #1940
• add caching copy layers back #1518 (introduces this wording)

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