builder: allow the USER directive to modify the owner of COPY'd files

This patch ensures that the USER directive will change the owner of any
COPY'd files or directories to the user id and primary group id of the
user which the builder's run config currently runs as. The user and
primary group ids are ascertained by inspecting the latest container's
/etc/passwd and /etc/group file (which is parsed on each COPY directive).

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