gentoo: move to /var/db/repos/gentoo [part 2]

gentoo docker image is fixed upstream
see https://github.com/gentoo/gentoo-docker-images/issues/69

gentoo

 FROM gentoo/portage:latest as portage
 FROM gentoo/stage3-amd64:latest
 
-COPY --from=portage /usr/portage /var/db/repos/gentoo
+COPY --from=portage /var/db/repos/gentoo /var/db/repos/gentoo
 
 ENV ACCEPT_KEYWORDS="amd64" \
     EMERGE_DEFAULT_OPTS="--jobs=9 --load-average=8 --with-bdeps=y" \