docker build depends on subrepos, too

Makefile

@@ -94,7 +94,7 @@ deps-test:
 	$(PIP) install -r requirements_test.txt
 
 # Build docker image
-docker:
+docker: repo/tesseract repo/tesserocr
 	docker build \
 	--build-arg VCS_REF=$$(git rev-parse --short HEAD) \
 	--build-arg BUILD_DATE=$$(date -u +"%Y-%m-%dT%H:%M:%SZ") \