build: Pass proxy environment variables through to docker containers.

When building behind a proxy, build via 'cargo make' worked up to
the point of 'running Task: fetch-vendored' where it hung.

The reason was because necessary proxy environment variables
were not passed into the docker container's environment.

The fix here is to add these variables to the environment of the
docker container if they are set outside the container:
  http_proxy, https_proxy, no_proxy

tools/docker-go

@@ -54,10 +54,19 @@ DOCKER_RUN_ARGS="--network=host"
 
 parse_args "${@}"
 
+# Go accepts both lower and uppercase proxy variables, pass both through.
+proxy_env=( )
+for i in http_proxy https_proxy no_proxy HTTP_PROXY HTTPS_PROXY NO_PROXY; do
+   if [ -n "${!i}" ]; then
+      proxy_env[${#proxy_env[@]}]="--env=$i=${!i}"
+   fi
+done
+
 docker run --rm \
   -e GOPRIVATE='*' \
   -e GOCACHE='/tmp/.cache' \
   -e GOPATH='/tmp/go' \
+  "${proxy_env[@]}" \
   --user "$(id -u):$(id -g)" \
   ${DOCKER_RUN_ARGS} \
   -v "${GO_MOD_CACHE}":/tmp/go/pkg/mod \