Add support for Docker container build behind proxy

Change-Id: I380f7b20f04ad81b487ccc3cafe37a38360dcf00
diff --git a/containers/cord-apps/Makefile b/containers/cord-apps/Makefile
index 191e249..4b789c4 100644
--- a/containers/cord-apps/Makefile
+++ b/containers/cord-apps/Makefile
@@ -3,8 +3,16 @@
 VTN_IMAGE_NAME:=xosproject/cord-app-vtn
 NO_DOCKER_CACHE?=false
 
+BUILD_ARGS =
+ifdef http_proxy
+BUILD_ARGS += --build-arg http_proxy=${http_proxy}
+endif
+ifdef https_proxy
+BUILD_ARGS += --build-arg https_proxy=${https_proxy}
+endif
+
 .PHONY: build
-build: ; sudo docker build --no-cache=${NO_DOCKER_CACHE} --rm -t ${IMAGE_NAME} .
+build: ; sudo docker build --no-cache=${NO_DOCKER_CACHE} --rm -t ${IMAGE_NAME} ${BUILD_ARGS} .
 
 .PHONY: config
 config: ; sudo docker build --rm -t ${CONFIG_IMAGE_NAME} -f Dockerfile.config .