[sonic-slave]: SLAVE_TAG should be for both Dockerfile and Dockerfile…

….user (#950)

Should rebuild both Dockerfile and Dockerfile.user when any of the two
files change.

Makefile

@@ -13,7 +13,7 @@ $(shell rm -f .screen)
 MAKEFLAGS += -B
 
 SLAVE_BASE_TAG = $(shell shasum sonic-slave/Dockerfile | awk '{print substr($$1,0,11);}')
-SLAVE_TAG = $(shell shasum sonic-slave/Dockerfile.user | awk '{print substr($$1,0,11);}')
+SLAVE_TAG = $(shell cat sonic-slave/Dockerfile.user sonic-slave/Dockerfile | shasum | awk '{print substr($$1,0,11);}')
 SLAVE_BASE_IMAGE = sonic-slave-base
 SLAVE_IMAGE = sonic-slave-$(USER)