Dockerfile: Update Dockerfile and add Docker optimizations

Dockerfile

@@ -1,15 +1,27 @@
-FROM golang:1.8-alpine
+FROM golang:alpine3.6
 
 ENV GOPATH /go
-ENV USER root
 
-RUN apk update && apk add git make
+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 && \
+  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
 
-# pre-install known dependencies before the source, so we don't redownload them whenever the source changes
-RUN go get github.com/kardianos/govendor \
- && govendor get github.com/gohugoio/hugo
+USER hugo
 
-COPY . $GOPATH/src/github.com/gohugoio/hugo
+WORKDIR /site
+
+EXPOSE 1313
+
+ENTRYPOINT ["/usr/bin/dumb-init", "--", "hugo"]
+
+CMD [ "--help" ]
 
-RUN cd $GOPATH/src/github.com/gohugoio/hugo \
- 	&& make install test