Run `make build` only once

CONTRIBUTING.md

@@ -30,6 +30,7 @@ $ CHANNEL=stable sh get-docker.sh
 Fire up the container with the following.
 
 ```bash
+$ make build  # only the first time or when changing container
 $ make serve
 ```
 

Makefile

@@ -11,7 +11,7 @@ stop:
 	@docker stop $(NAME) && docker rm $(NAME) --force
 .PHONY: stop
 
-serve: build
+serve:
 	@docker run \
 		--name $(NAME) \
 		-e JEKYLL_PORT=$(PORT) \