Dockerfile: Optimize Docker image size

Reduce Docker image size from 428 MB to 277 MB.

See #3674

Dockerfile

@@ -4,24 +4,22 @@ ENV GOPATH /go
 
 RUN \
   adduser -h /site -s /sbin/nologin -u 1000 -D hugo && \
-  apk add --no-cache dumb-init && \
-  apk add --no-cache --virtual .build-deps \
-    git \
-    make && \
+  apk add --no-cache \
+    dumb-init \
+    git && \
   go get github.com/kardianos/govendor && \
   govendor get github.com/gohugoio/hugo && \
   cd $GOPATH/src/github.com/gohugoio/hugo && \
-  make install test && \
-  rm -rf $GOPATH/src/* && \
-  apk del .build-deps
-
-USER hugo
+  go install && \
+  cd $GOPATH && \
+  rm -rf pkg src .cache bin/govendor && \
+  apk del --no-cache git go
 
+USER    hugo
 WORKDIR /site
-
-EXPOSE 1313
+VOLUME  /site
+EXPOSE  1313
 
 ENTRYPOINT ["/usr/bin/dumb-init", "--", "hugo"]
-
 CMD [ "--help" ]