Adjust dockerfile and docker-build steps

- Now that sourcing requires, board/ for the bsp, we need to
pull this into the container. So copy board/build into a temporary
folder, and place the Dockerfile alongside them. The rest of the setup
we run to make the container is unchanged.
- Add some dependencies to docker-build target, so that it properly
regenerates if a new prerequisite or the like is added.

Change-Id: I56afdc5fcef52f4029e1745c815265a0bfc8ed9b
diff --git a/Dockerfile b/Dockerfile
index 68c6e53..cf0c189 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -16,7 +16,8 @@
 # Install the prerequisite packages into the image.
-ADD . /build
+ADD ./build /build
+ADD ./board /board
 RUN /bin/bash -c '\
 apt-get update && \
 apt-get install sudo make && \
diff --git a/ b/
index 4a3767e..66fabf1 100644
--- a/
+++ b/
@@ -26,11 +26,15 @@
 	mkdir -p $(ROOTDIR)/cache
 	cp $< $(ROOTDIR)/cache
-	mkdir -p $(ROOTDIR)/cache
-	docker build -t aiy-board-builder $(ROOTDIR)/build
+$(ROOTDIR)/cache/aiy-board-builder.tar: $(ROOTDIR)/build/Dockerfile $(ROOTDIR)/build/
+	mkdir -p $(ROOTDIR)/cache/docker-build
+	cp -a $(ROOTDIR)/build $(ROOTDIR)/cache/docker-build
+	cp -a $(ROOTDIR)/board $(ROOTDIR)/cache/docker-build
+	cp -a $(ROOTDIR)/build/Dockerfile $(ROOTDIR)/cache/docker-build
+	docker build -t aiy-board-builder $(ROOTDIR)/cache/docker-build
 	docker image save -o $@ aiy-board-builder:latest
 	docker rmi aiy-board-builder:latest
+	rm -rf $(ROOTDIR)/cache/docker-build
 # Runs any make TARGET in x86 docker image ('m docker-TARGET')