if ftp_proxy/http_proxy/https_proxy standard environment variables available,
pass them to the docker daemon to build images.
this is required when building behind corporate proxy/firewall, but also help
when using local cache server (ie: apt/yum).

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---

v2: use Fam Zheng's pythonized one-line,
    renamed env vars to lowercase.

debian builds can be tested using Sameer Naik docker image
(see https://github.com/sameersbn/docker-apt-cacher-ng#quickstart)

$ docker run --name apt-cacher-ng -d --restart=always \
  --publish 127.0.0.1:3142:3142 \
  --volume /var/cache/apt-cacher-ng:/var/cache/apt-cacher-ng \
  --privileged \
  sameersbn/apt-cacher-ng:latest

$ HTTP_PROXY=http://172.17.0.1:3142 make docker-test-build@debian-armhf-cross

 tests/docker/docker.py | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tests/docker/docker.py b/tests/docker/docker.py
index 9fd32ab5fa..8747f6a440 100755
--- a/tests/docker/docker.py
+++ b/tests/docker/docker.py
@@ -28,6 +28,9 @@ from shutil import copy, rmtree
 from pwd import getpwuid
 
 
+FILTERED_ENV_NAMES = ['ftp_proxy', 'http_proxy', 'https_proxy']
+
+
 DEVNULL = open(os.devnull, 'wb')
 
 
@@ -272,6 +275,9 @@ class BuildCommand(SubCommand):
                 _copy_binary_with_libs(args.include_executable,
                                        docker_dir)
 
+            argv += ["--build-arg=" + k.lower() + "=" + v
+                        for k, v in os.environ.iteritems()
+                        if k.lower() in FILTERED_ENV_NAMES]
             dkr.build_image(tag, docker_dir, dockerfile,
                             quiet=args.quiet, user=args.user, argv=argv)
 
-- 
2.11.0