Use progressive build to reduce final image size

- Use progressive build to reduce final image size
- Download source during build so only Dockerfile is required to be present on the build host
- Add a `version` arg so any tag or branch can be built: `docker build --build-arg=version=v0.25.0 -t wtf .`

Dockerfile

@@ -1,8 +1,11 @@
-FROM golang:1.13-alpine
+FROM golang:1.13-alpine as build
 
-RUN apk add --no-cache make ncurses
+ARG version=master
 
-COPY . $GOPATH/src/github.com/wtfutil/wtf
+RUN apk add git make ncurses && \
+    git clone https://github.com/wtfutil/wtf.git $GOPATH/src/github.com/wtfutil/wtf && \
+    cd $GOPATH/src/github.com/wtfutil/wtf && \
+    git checkout $version
 
 ENV GOPROXY=https://proxy.golang.org,direct
 ENV GO111MODULE=on
@@ -14,4 +17,6 @@ ENV PATH=$PATH:./bin
 
 RUN make build
 
+FROM alpine
+COPY --from=build /go/src/github.com/wtfutil/wtf/bin/wtfutil /usr/local/bin/
 ENTRYPOINT "wtfutil"

README.md

@@ -122,12 +122,15 @@ make run
 You can run `wtf` inside a docker container:
 
 ```bash
-# download the source
-git clone https://github.com/wtfutil/wtf
+# download or create the Dockerfile
+curl -o Dockerfile https://raw.githubusercontent.com/wtfutil/wtf/master/Dockerfile
 
 # build the docker container
 docker build -t wtfutil .
 
+# or for a particular tag or branch
+docker build --build-args=version=v0.25.0 -t wtfutil .
+
 # run the container
 docker run -it wtfutil
 ```