build: pass Go module proxy variables through docker-go

Previously, we always set `GOPRIVATE=*`, which blocked any use of the
public Go module mirror.

Now we pass the module proxy variables through from the environment,
so that the behavior is under the developer's control.

tools/docker-go

@@ -54,18 +54,31 @@ DOCKER_RUN_ARGS="--network=host"
 
 parse_args "${@}"
 
+# Pass through relevant Go variables, from the config or environment.
+go_env=( )
+for i in GOPROXY GONOPROXY GOPRIVATE ; do
+  if command -v go >/dev/null 2>&1 ; then
+    govar="$(go env ${i})"
+    if [ -n "${govar}" ] ; then
+      go_env[${#go_env[@]}]="--env=${i}=${govar}"
+    fi
+  elif [ -n "${!i}" ] ; then
+    go_env[${#go_env[@]}]="--env=${i}=${!i}"
+  fi
+done
+
 # Go accepts both lower and uppercase proxy variables, pass both through.
 proxy_env=( )
-for i in http_proxy https_proxy no_proxy HTTP_PROXY HTTPS_PROXY NO_PROXY; do
-   if [ -n "${!i}" ]; then
-      proxy_env[${#proxy_env[@]}]="--env=$i=${!i}"
-   fi
+for i in http_proxy https_proxy no_proxy HTTP_PROXY HTTPS_PROXY NO_PROXY ; do
+  if [ -n "${!i}" ]; then
+    proxy_env[${#proxy_env[@]}]="--env=$i=${!i}"
+  fi
 done
 
 docker run --rm \
-  -e GOPRIVATE='*' \
   -e GOCACHE='/tmp/.cache' \
   -e GOPATH='/tmp/go' \
+  "${go_env[@]}" \
   "${proxy_env[@]}" \
   --user "$(id -u):$(id -g)" \
   --security-opt label:disable \