Set build arg values in docker.Makefile

Sets the values of the --build-args passed through to docker build from
vars.mk when using make with docker.Makefile. This will correctly set
the TAG so that the binary versions are set as when using the standard
Makefile directly

Signed-off-by: Nick Adcock <nick.adcock@docker.com>

Author: zappy-shu

docker.Makefile

@@ -14,7 +14,7 @@ E2E_CROSS_CTNR_NAME := $(BIN_NAME)-e2e-cross-$(TAG)
 COV_CTNR_NAME := $(BIN_NAME)-cov-$(TAG)
 SCHEMAS_CTNR_NAME := $(BIN_NAME)-schemas-$(TAG)
 
-BUILD_ARGS=--build-arg=EXPERIMENTAL --build-arg=TAG --build-arg=COMMIT --build-arg=ALPINE_VERSION --build-arg=GOPROXY
+BUILD_ARGS=--build-arg EXPERIMENTAL=$(EXPERIMENTAL) --build-arg TAG=$(TAG) --build-arg COMMIT=$(COMMIT) --build-arg ALPINE_VERSION=$(ALPINE_VERSION) --build-arg GOPROXY=$(GOPROXY)
 
 PKG_PATH := /go/src/$(PKG_NAME)