Makefiles: build the test docker img

this commit changes how the dockertest image is built.
it moves the command into dockertest/Makefile. It also
uses a cached file that -- if removed with make clean
-- can signal whether the image should be rebuilt.
(it may be ideal to have it either detect code has
changed, or just rebuild every time. )

Makefile

@@ -18,7 +18,6 @@ test: test_go test_sharness
 test_expensive: test_go_expensive test_sharness_expensive
 
 test_docker:
-	docker build -t zaqwsx_ipfs-test-img .
 	cd dockertest/ && make
 
 test_go:

dockertest/Makefile

@@ -14,9 +14,6 @@ setup: docker_ipfs_image data/filetiny data/filerand
 save_logs:
 	sh bin/save_logs.sh
 
-docker_ipfs_image:
-	docker images | grep $(IPFS_DOCKER_IMAGE)
-
 data/filetiny: Makefile
 	cp Makefile ./data/filetiny # simple
 
@@ -26,6 +23,16 @@ data/filerand: bin/random
 bin/random:
 	go build -o ./bin/random ../$(RANDOMSRC)
 
+docker_ipfs_image: build/.built_img
+	docker images | grep $(IPFS_DOCKER_IMAGE)
+
+# this is here so that "make clean; make" rebuilds the docker img.
+# but otherwise will reuse whatever you last built.
+# TODO: make this detect whether code has been changed
+build/.built_img:
+	cd .. && docker build -t $(IPFS_DOCKER_IMAGE) .
+	touch build/.built_img
+
 clean:
 	sh bin/clean.sh
 	fig stop
@@ -34,3 +41,4 @@ clean:
 	rm -f data/filetiny
 	rm -f data/filerand
 	rm -rf build/*
+	rm -rf build/.built_img

dockertest/build/.gitignore

-*.log
-go-random
+.built_img