Make .ensure-syscall-test to use build-arg

.ensure-syscall-test need to build an image, we have
to add http_proxy and https_proxy to make it work.
Inseted of adding http_proxy and https_proxy env to the Dockerfile,
adding a DOCKER_BUILD_ARGS env to pass the build-arg is much more
elegant.

Signed-off-by: Lei Jitang <leijitang@huawei.com>