Add DOCKER_BUILD_FLAGS environment variable

This environment variable can be use to add some flags to the docker
build command.

For example it allow the build of docker images when the developper is
behind a http proxy by using the '--build-arg http_proxy --build-arg
https_proxy' flags.

Makefile

@@ -42,6 +42,9 @@ GOLDFLAGS := -ldflags "-X $(PACKAGE_NAME)/pkg/util.AppGitState=${GIT_STATE} -X $
 	$(CMDS) go_test go_fmt e2e_test go_verify hack_verify hack_verify_pr \
 	$(DOCKER_BUILD_TARGETS) $(DOCKER_PUSH_TARGETS)
 
+# Docker build flags
+DOCKER_BUILD_FLAGS := --build-arg VCS_REF=$(GIT_COMMIT) $(DOCKER_BUILD_FLAGS)
+
 # Alias targets
 ###############
 
@@ -127,7 +130,7 @@ e2e_test:
 $(DOCKER_BUILD_TARGETS):
 	$(eval DOCKER_BUILD_CMD := $(subst docker_build_,,$@))
 	docker build \
-		--build-arg VCS_REF=$(GIT_COMMIT) \
+		$(DOCKER_BUILD_FLAGS) \
 		-t $(REGISTRY)/$(APP_NAME)-$(DOCKER_BUILD_CMD):$(BUILD_TAG) \
 		-f $(DOCKERFILES)/$(DOCKER_BUILD_CMD)/Dockerfile \
 		$(DOCKERFILES)