docker: add Dockerfile for building a checkup docker image

Dockerfile

@@ -0,0 +1,15 @@
+FROM golang:1.13-alpine as builder
+
+ENV CGO_ENABLED=0
+
+COPY . /app
+WORKDIR /app
+RUN apk --no-cache add make && make build
+
+FROM alpine:latest
+
+WORKDIR /app
+
+COPY --from=builder /app/builds/checkup /usr/local/bin/checkup
+
+ENTRYPOINT ["checkup"]

Makefile

@@ -1,6 +1,8 @@
 .PHONY: all build test docker
 
-all: build test docker
+all: build test
+
+DOCKER_IMAGE := checkup
 
 build:
 	mkdir -p builds/
@@ -10,4 +12,4 @@ test:
 	go test -race -count=1 -v ./...
 
 docker:
-	docker build --no-cache . -t checkup
+	docker build --no-cache . -t $(DOCKER_IMAGE)