Set GOFLAGS default environment variable in Makefile to enable overwr… (#870)

* Set GOFLAGS default environment variable in Makefile to enable overwrites
* Fix quotation to enable to pass in multiple flags

Author: martialonline

Makefile

@@ -7,6 +7,7 @@ PREFIX = nginx/nginx-ingress
 DOCKER_TEST_RUN = docker run --rm -v $(shell pwd):/go/src/github.com/nginxinc/kubernetes-ingress -w /go/src/github.com/nginxinc/kubernetes-ingress
 DOCKER_BUILD_RUN = docker run --rm -v $(shell pwd):/go/src/github.com/nginxinc/kubernetes-ingress -w /go/src/github.com/nginxinc/kubernetes-ingress/cmd/nginx-ingress/
 GOLANG_CONTAINER = golang:1.13
+GOFLAGS ?= -mod=vendor
 DOCKERFILEPATH = build
 DOCKERFILE = Dockerfile # note, this can be overwritten e.g. can be DOCKERFILE=DockerFileForPlus
 
@@ -19,19 +20,19 @@ GIT_COMMIT=$(shell git rev-parse --short HEAD)
 
 nginx-ingress:
 ifeq ($(BUILD_IN_CONTAINER),1)
-	$(DOCKER_BUILD_RUN) -e CGO_ENABLED=0 -e GO111MODULE=on -e GOFLAGS='-mod=vendor' $(GOLANG_CONTAINER) go build -installsuffix cgo -ldflags "-w -X main.version=${VERSION} -X main.gitCommit=${GIT_COMMIT}" -o /go/src/github.com/nginxinc/kubernetes-ingress/nginx-ingress
+	$(DOCKER_BUILD_RUN) -e CGO_ENABLED=0 -e GO111MODULE=on -e GOFLAGS='$(GOFLAGS)' $(GOLANG_CONTAINER) go build -installsuffix cgo -ldflags "-w -X main.version=${VERSION} -X main.gitCommit=${GIT_COMMIT}" -o /go/src/github.com/nginxinc/kubernetes-ingress/nginx-ingress
 else
-	CGO_ENABLED=0 GO111MODULE=on GOFLAGS='-mod=vendor' GOOS=linux go build -installsuffix cgo -ldflags "-w -X main.version=${VERSION} -X main.gitCommit=${GIT_COMMIT}" -o nginx-ingress github.com/nginxinc/kubernetes-ingress/cmd/nginx-ingress
+	CGO_ENABLED=0 GO111MODULE=on GOFLAGS='$(GOFLAGS)' GOOS=linux go build -installsuffix cgo -ldflags "-w -X main.version=${VERSION} -X main.gitCommit=${GIT_COMMIT}" -o nginx-ingress github.com/nginxinc/kubernetes-ingress/cmd/nginx-ingress
 endif
 
 lint:
 	golangci-lint run
 
 test:
 ifeq ($(BUILD_IN_CONTAINER),1)
-	$(DOCKER_TEST_RUN) -e GO111MODULE=on -e GOFLAGS='-mod=vendor' $(GOLANG_CONTAINER) go test ./...
+	$(DOCKER_TEST_RUN) -e GO111MODULE=on -e GOFLAGS='$(GOFLAGS)' $(GOLANG_CONTAINER) go test ./...
 else
-	GO111MODULE=on GOFLAGS='-mod=vendor' go test ./...
+	GO111MODULE=on GOFLAGS='$(GOFLAGS)' go test ./...
 endif
 
 verify-codegen: