Dockerfile --build-arg - behaviour of http_proxy

test_http_proxy_in_dockerfile.sh

#!/bin/sh -x

# test using your own http_proxy value at build time
# there is a default ARG for http_proxy already available

IMG=test

docker rm -f ${IMG}
cat <<EOF | docker build  --build-arg HTTP_PROXY=http://piers-work:3128 -t ${IMG} -

FROM ubuntu:18.04

ARG HTTP_PROXY=""
RUN echo "here is http_proxy: \$HTTP_PROXY"

ENV HOME /root
ENV DEBIAN_FRONTEND noninteractive

RUN apt-get update && apt-get install curl -y && apt-get clean -y && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

RUN export https_proxy=\$HTTP_PROXY && curl -vvv https://ska-registry.av.it.pt
EOF