Update Go version locking

Add a mechanism to trigger an image build in case Go gmt gets an update.

Author: pointlessone

Dockerfile

@@ -1,18 +1,28 @@
-FROM golang:alpine
+ARG BASE=1.9.3-alpine3.7
+FROM golang:${BASE} as build
 
+WORKDIR /usr/src/app
+
+COPY engine.json ./engine.json.template
+RUN apk add --no-cache jq
+RUN export go_version=$(go version | cut -d ' ' -f 3) && \
+    cat engine.json.template | jq '.version = .version + "/" + env.go_version' > ./engine.json
+
+COPY codeclimate-gofmt.go ./
+RUN apk add --no-cache git
+RUN go get -t -d -v .
+RUN go build -o codeclimate-gofmt .
+
+
+FROM golang:${BASE}
 LABEL maintainer="Code Climate <hello@codeclimate.com>"
 
 RUN adduser -u 9000 -D app
 
 WORKDIR /usr/src/app
 
-COPY engine.json /engine.json
-COPY codeclimate-gofmt.go ./
-RUN apk add --no-cache --virtual .dev-deps git && \
-  go get -t -d -v . && \
-  go build -o codeclimate-gofmt . && \
-  rm -r $GOPATH/src/* && \
-  apk del .dev-deps
+COPY --from=build /usr/src/app/engine.json /
+COPY --from=build /usr/src/app/codeclimate-gofmt ./
 
 USER app
 

Makefile

@@ -1,14 +1,6 @@
-.PHONY: update image
+.PHONY: image
 
 IMAGE_NAME ?= codeclimate/codeclimate-gofmt
 
-update:
-	docker run \
-	  --rm --interactive \
-	  -v $(PWD)/engine.json:/engine.json \
-	  -v $(PWD)/bin/update:/usr/local/bin/update \
-	  golang:alpine \
-	  /usr/local/bin/update
-
 image:
 	docker build --tag "$(IMAGE_NAME)" .

README.md

@@ -27,15 +27,10 @@ This will build a `codeclimate/codeclimate-gofmt` image locally.
 ### Updating
 
 `gofmt` is a part of the Go distribution and shares version with it. Once in a
-while a new version of Go gets packaged and in order to get the latest version
-and force a new docker image build, we have to update the `engine.json` file.
-
-```console
-make update
-```
-
-This will update engine version with the latest `gofmt` version. After that
-rebuild the image as usually.
+while a new version of Go gets packaged. In order to get the latest version
+and force a new docker image build, please update the base image in the
+`Dockerfile`. Please avoid any unstable tags such as `latest` and keep it
+explicit.
 
 ### Need help?
 

bin/update

@@ -8,6 +8,6 @@
   "languages": [
     "Go"
   ],
-  "version": "1.0.0/go-1.8.4-r0",
+  "version": "1.0.0",
   "spec_version": "0.3.1"
 }