build-rootfs-size-docker: Enable proxy

Add http_proxy and UBUNTU_MIRROR support as build-setup.sh, so that it
is possible to make the script work behind a proxy and use a different
ubuntu mirror.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I833d24e3f3ccef19abfb01cdb9c62b3d8a195b35
1 file changed