[build]: only build dockers needed by installers (#2771)

Signed-off-by: Guohan Lu <gulv@microsoft.com>

slave.mk

@@ -472,8 +472,10 @@ SONIC_TARGET_LIST += $(addprefix $(TARGET_PATH)/, $(SONIC_SIMPLE_DOCKER_IMAGES))
 
 # Build stretch docker images only in stretch slave docker,
 # jessie docker images only in jessie slave docker
+DOCKER_IMAGES_FOR_INSTALLERS := $(sort $(foreach installer,$(SONIC_INSTALLERS),$($(installer)_DOCKERS)))
 ifeq ($(BLDENV),stretch)
 	DOCKER_IMAGES := $(SONIC_STRETCH_DOCKERS)
+	SONIC_STRETCH_DOCKERS_FOR_INSTALLERS = $(filter $(SONIC_STRETCH_DOCKERS),$(DOCKER_IMAGES_FOR_INSTALLERS))
 else
 	DOCKER_IMAGES := $(filter-out $(SONIC_STRETCH_DOCKERS), $(SONIC_DOCKER_IMAGES))
 endif
@@ -681,7 +683,7 @@ all : .platform $$(addprefix $(TARGET_PATH)/,$$(SONIC_ALL))
 
 stretch : $$(addprefix $(DEBS_PATH)/,$$(SONIC_STRETCH_DEBS)) \
           $$(addprefix $(FILES_PATH)/,$$(SONIC_STRETCH_FILES)) \
-          $$(addprefix $(TARGET_PATH)/,$$(SONIC_STRETCH_DOCKERS))
+          $$(addprefix $(TARGET_PATH)/,$$(SONIC_STRETCH_DOCKERS_FOR_INSTALLERS))
 
 
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