Dockerfile: Reduce image size from 277MB to 27MB

"xtrem" awesome container size optimization
by using alpine:3.6 as base image
and by installing Go at build time.

See #3730 and #3738

Dockerfile

@@ -1,19 +1,28 @@
-FROM golang:alpine3.6
+FROM alpine:3.6
 
 ENV GOPATH /go
+ENV PATH $GOPATH/bin:$PATH
 
 RUN \
   adduser -h /site -s /sbin/nologin -u 1000 -D hugo && \
   apk add --no-cache \
-    dumb-init \
+    dumb-init && \
+  apk add --no-cache --virtual .build-deps \
+    gcc \
+    musl-dev \
+    go \
     git && \
+  mkdir -p \
+    ${GOPATH}/bin \
+    ${GOPATH}/pkg \
+    ${GOPATH}/src && \
   go get github.com/kardianos/govendor && \
   govendor get github.com/gohugoio/hugo && \
   cd $GOPATH/src/github.com/gohugoio/hugo && \
   go install && \
   cd $GOPATH && \
   rm -rf pkg src .cache bin/govendor && \
-  apk del --no-cache git go
+  apk del .build-deps
 
 USER    hugo
 WORKDIR /site
@@ -22,4 +31,3 @@ EXPOSE  1313
 
 ENTRYPOINT ["/usr/bin/dumb-init", "--", "hugo"]
 CMD [ "--help" ]
-