Dockerfile: use alpine:3.6, clean up apk incovation (#15006)

Dockerfile

@@ -1,15 +1,13 @@
-FROM alpine:3.5
+FROM alpine:3.6
 
 ADD . /go-ethereum
 RUN \
-  apk add --update git go make gcc musl-dev linux-headers && \
+  apk add --no-cache git go make gcc musl-dev linux-headers && \
   (cd go-ethereum && make geth)                             && \
   cp go-ethereum/build/bin/geth /usr/local/bin/             && \
   apk del git go make gcc musl-dev linux-headers            && \
-  rm -rf /go-ethereum && rm -rf /var/cache/apk/*
+  rm -rf /go-ethereum
 
-EXPOSE 8545
-EXPOSE 30303
-EXPOSE 30303/udp
+EXPOSE 8545 30303 30303/udp
 
 ENTRYPOINT ["geth"]