Uses multi-stage builds to create images

follows along here loosely:

https://docs.docker.com/develop/develop-images/multistage-build/
https://blog.alexellis.io/mutli-stage-docker-builds/

Makefile

@@ -1,19 +1,20 @@
 GOFILES := $(shell find . -type f | grep go$$ )
 
-build/lim: build ${GOFILES}
-	@docker build -t smcquay/lim:build . -f Dockerfile.build > /dev/null
-	@docker create --name ext smcquay/lim:build > /dev/null
-	@docker cp ext:/usr/local/bin/lim build/ > /dev/null
+build/lim: build build/.docker-image ${GOFILES}
+	@echo "fetching binary from image"
+	@docker create --name ext smcquay/lim:latest > /dev/null
+	@docker cp ext:/bin/lim build/ > /dev/null
 	@docker rm -f ext > /dev/null
-	@docker image rm smcquay/lim:build > /dev/null
+	@touch build/lim
+
+build/.docker-image: ${GOFILES}
+	@echo "building docker image"
+	@docker build --no-cache -t smcquay/lim:latest . > /dev/null
+	@touch build/.docker-image
 
 build:
 	@mkdir build
 
-.PHONY: docker-image
-docker-image: build/lim
-	@docker build --no-cache -t smcquay/lim:latest  . > /dev/null
-
 .PHONY: clean
 clean:
 	@rm -rf build