Pass FETCH_PACKAGES setting to Docker

-- Otherwise, always stuck building the long way...

Change-Id: I145cdbf1bfea66b677d25f5f664cc4b7f23c6564
diff --git a/docker.mk b/docker.mk
index be694f2..bb60444 100644
--- a/docker.mk
+++ b/docker.mk
@@ -59,6 +59,7 @@
 		-e "PREBUILT_MODULES_ROOT=/modules" \
 		-e "FETCH_PBUILDER_DIRECTORY=/pbuilder" \
 		-e "PACKAGES_FETCH_ROOT_DIRECTORY=/packages" \
+		-e "FETCH_PACKAGES=$(FETCH_PACKAGES)" \
 		aiy-board-builder \
 		/bin/bash -c \
 			'groupadd --gid $(shell id -g) $(shell id -g -n); \