makefile: use host GOPROXY value for %-in-docker

The host may have a value defined for GOPROXY that should still be
respected inside containers used for building Go programs.

Signed-off-by: Samuel Karp <[email protected]>

Author: samuelkarp

Makefile

@@ -62,6 +62,7 @@ $(SUBDIRS):
 		--env GOPATH=/go \
 		--env GO111MODULES=on \
 		--env STATIC_AGENT=on \
+		--env GOPROXY=$(shell go env GOPROXY) \
 		--workdir /src \
 		$(FIRECRACKER_CONTAINERD_BUILDER_IMAGE) \
 		$(MAKE) $(subst -in-docker,,$@)