Don't calculate images twice.

docker.mk

@@ -5,6 +5,8 @@ SHARDS=1
 
 dockerfiles:=$(shell ls docker/build/*/Dockerfile)
 all_images:=$(patsubst docker/build/%/Dockerfile,%,$(dockerfiles))
+
+# Used in the test.mk file as well.
 images:=$(if $(TRAVIS_COMMIT_RANGE),$(shell git diff --name-only $(TRAVIS_COMMIT_RANGE) | python util/parsefiles.py),$(all_images))
 
 docker_build=docker.build.

test.mk

@@ -2,7 +2,7 @@
 yml_files:=$(shell find . -name "*.yml")
 json_files:=$(shell find . -name "*.json")
 jinja_files:=$(shell find . -name "*.j2")
-images = $(if $(TRAVIS_COMMIT_RANGE), $(shell git diff --name-only $(TRAVIS_COMMIT_RANGE) | python util/parsefiles.py), $(all_images))
+# $(images) is calculated in the docker.mk file
 
 test: test.syntax test.edx_east_roles