Often when building a Docker image, the Dockerfile contains instructions that runs programs to fetch resources from the Internet (
wget for example to pull a program binary build on GitHub for example).
It is possible to instruct Docker to pass set set environment variables so that such programs perform those fetches through a proxy:
$ docker build --build-arg http_proxy=http://myproxy.example.com:3128 \ --build-arg https_proxy=http://myproxy.example.com:3128 \ --build-arg no_proxy=internal.example.com \ -t test .
build-arg are environment variables which are available at build time only.