use prebuilt images instead of cloning the repo

[nikhiljha]

metabase/Makefile

Lines 13 to 17 in 7a96cca

	cook-image:
	git clone https://github.com/metabase/metabase --depth 1 -b $(MB_VERSION) src
	cd src; git apply ../patches/*; \
	docker build --pull -t $(DOCKER_TAG) .
	rm -rf src

Metabase has official tagged images in Docker hub. The branch the current makefile uses doesn't look like it exists anymore, so maybe we should switch. https://hub.docker.com/r/metabase/metabase/tags