docs: update docs referring to COPY and USER to reflect UX change

This patch changes the documentation that refer to the COPY and USER
instructions, so that they reflect the new UX of COPY and USER (that
COPY copies files and chowns them with the same runconfig as the
currently set USER -- with a default of 0:0).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)